Add READMEs for ES UI plugins (#81973)

* Add Search Profiler README.
* Add Upgrade Assistant README.
* Add name of plugin to Watcher README.
* Add Console Extensions README.
* Add Grok Debugger README.
* Add Painless Lab README.
* Add License Management README.
* Add Remote Clusters README.
* Add Console README.

docs/developer/plugin-list.asciidoc

@@ -38,8 +38,8 @@ NOTE:
 |The Charts plugin is a way to create easier integration of shared colors, themes, types and other utilities across all Kibana charts and visualizations.
 
 
-|{kib-repo}blob/{branch}/src/plugins/console[console]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/src/plugins/console/README.md[console]
+|Console provides the user with tools for storing and executing requests against Elasticsearch.
 
 
 |<<kibana-dashboard-plugin>>
@@ -307,8 +307,8 @@ Failure to have auth enabled in Kibana will make for a broken UI. UI-based error
 |WARNING: Missing README.
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/console_extensions[consoleExtensions]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/console_extensions/README.md[consoleExtensions]
+|This plugin provides autocomplete definitions of licensed APIs to the OSS Console plugin.
 
 
 |{kib-repo}blob/{branch}/x-pack/plugins/cross_cluster_replication/README.md[crossClusterReplication]
@@ -376,8 +376,9 @@ or dashboards from the Kibana instance, from both server and client-side plugins
 |This is the main source folder of the Graph plugin. It contains all of the Kibana server and client source code. x-pack/test/functional/apps/graph contains additional functional tests.
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/grokdebugger[grokdebugger]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/grokdebugger/README.md[grokdebugger]
+|This plugin helps users define Grok patterns,
+which are particularly useful for ingesting logs.
 
 
 |{kib-repo}blob/{branch}/x-pack/plugins/index_lifecycle_management/README.md[indexLifecycleManagement]
@@ -406,8 +407,8 @@ the infrastructure monitoring use-case within Kibana.
 |Run all tests from the x-pack root directory
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/license_management[licenseManagement]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/license_management/README.md[licenseManagement]
+|This plugin enables users to activate a trial license, downgrade to Basic, and upload a new license.
 
 
 |{kib-repo}blob/{branch}/x-pack/plugins/licensing/README.md[licensing]
@@ -444,12 +445,12 @@ Elastic.
 |This plugin provides shared components and services for use across observability solutions, as well as the observability landing page UI.
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/painless_lab[painlessLab]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/painless_lab/README.md[painlessLab]
+|This plugin helps users learn how to use the Painless scripting language.
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/remote_clusters[remoteClusters]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/remote_clusters/README.md[remoteClusters]
+|This plugin helps users manage their remote clusters, which enable cross-cluster search and cross-cluster replication.
 
 
 |{kib-repo}blob/{branch}/x-pack/plugins/reporting/README.md[reporting]
@@ -460,8 +461,11 @@ Elastic.
 |Welcome to the Kibana rollup plugin! This plugin provides Kibana support for Elasticsearch's rollup feature. Please refer to the Elasticsearch documentation to understand rollup indices and how to create rollup jobs.
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/searchprofiler[searchprofiler]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/searchprofiler/README.md[searchprofiler]
+|The search profiler consumes the Profile API
+by sending a search API with profile: true enabled in the request body. The response contains
+detailed information on how Elasticsearch executed the search request. People use this information
+to understand why a search request might be slow.
 
 
 |{kib-repo}blob/{branch}/x-pack/plugins/security/README.md[security]
@@ -513,8 +517,9 @@ As a developer you can reuse and extend built-in alerts and actions UI functiona
 |Registers commercially licensed generic actions like per panel time range and contains some code that supports drilldown work.
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/upgrade_assistant[upgradeAssistant]
-|WARNING: Missing README.
+|{kib-repo}blob/{branch}/x-pack/plugins/upgrade_assistant/README.md[upgradeAssistant]
+|Upgrade Assistant helps users prepare their Stack for being upgraded to the next major. Its primary
+purposes are to:
 
 
 |{kib-repo}blob/{branch}/x-pack/plugins/uptime/README.md[uptime]

src/plugins/console/README.md

@@ -0,0 +1,5 @@
+# Console
+
+## About
+
+Console provides the user with tools for storing and executing requests against Elasticsearch.

x-pack/plugins/console_extensions/README.md

@@ -0,0 +1,3 @@
+# Console extensions
+
+This plugin provides autocomplete definitions of licensed APIs to the OSS Console plugin.

x-pack/plugins/grokdebugger/README.md

@@ -0,0 +1,6 @@
+# Grok Debugger
+
+## About
+
+This plugin helps users define [Grok patterns](https://www.elastic.co/guide/en/elasticsearch/reference/current/grok-processor.html),
+which are particularly useful for ingesting logs.

x-pack/plugins/license_management/README.md

@@ -0,0 +1,5 @@
+# License Management
+
+## About
+
+This plugin enables users to activate a trial license, downgrade to Basic, and upload a new license.

x-pack/plugins/painless_lab/README.md

@@ -0,0 +1,5 @@
+# Painless Lab
+
+## About
+
+This plugin helps users learn how to use the [Painless scripting language](https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-scripting-painless.html).

x-pack/plugins/remote_clusters/README.md

@@ -0,0 +1,5 @@
+# Remote Clusters
+
+## About
+
+This plugin helps users manage their [remote clusters](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-remote-clusters.html), which enable cross-cluster search and cross-cluster replication.

x-pack/plugins/searchprofiler/README.md

@@ -0,0 +1,8 @@
+# Search Profiler
+
+## About
+
+The search profiler consumes the [Profile API](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-profile.html)
+by sending a `search` API with `profile: true` enabled in the request body. The response contains
+detailed information on how Elasticsearch executed the search request. People use this information
+to understand why a search request might be slow.

x-pack/plugins/upgrade_assistant/README.md

@@ -0,0 +1,67 @@
+# Upgrade Assistant
+
+## About
+
+Upgrade Assistant helps users prepare their Stack for being upgraded to the next major. Its primary
+purposes are to:
+
+* **Surface deprecations.** Deprecations are features that are currently being used that will be
+removed in the next major. Surfacing tells the user that there's a problem preventing them
+from upgrading.
+* **Migrate from deprecation features to supported features.** This addresses the problem, clearing
+the path for the upgrade. Generally speaking, once all deprecations are addressed, the user can
+safely upgrade.
+
+### Deprecations
+
+There are two sources of deprecation information:
+
+* [**Deprecation Info API.**](https://www.elastic.co/guide/en/elasticsearch/reference/master/migration-api-deprecation.html)
+This is information about cluster, node, and index level settings that use deprecated features that
+will be removed or changed in the next major version. Currently, only cluster and index deprecations
+will be surfaced in the Upgrade Assistant. ES server engineers are responsible for adding
+deprecations to the Deprecation Info API.
+* [**Deprecation logs.**](https://www.elastic.co/guide/en/elasticsearch/reference/current/logging.html#deprecation-logging)
+These surface runtime deprecations, e.g. a Painless script that uses a deprecated accessor or a
+request to a deprecated API. These are also generally surfaced as deprecation headers within the
+response. Even if the cluster state is good, app maintainers need to watch the logs in case
+deprecations are discovered as data is migrated.
+
+### Fixing problems
+
+Problems can be fixed at various points in the upgrade process. The Upgrade Assistant supports
+various upgrade paths and surfaces various types of upgrade-related issues.
+
+* **Fixing deprecated cluster settings pre-upgrade.** This generally requires fixing some settings
+in `elasticsearch.yml`.
+* **Migrating indices data pre-upgrade.** This can involve deleting indices so that ES can rebuild
+them in the new version, reindexing them so that they're built using a new Lucene version, or
+applying a migration script that reindexes them with new settings/mappings/etc.
+* **Migrating indices data post-upgrade.** As was the case with APM in the 6.8->7.x upgrade,
+sometimes the new data format isn't forwards-compatible. In these cases, the user will perform the
+upgrade first and then use the Upgrade Assistant to reindex their data to be compatible with the new
+version.
+
+Deprecations can be handled in a number of ways:
+
+* **Reindexing.** When a user's index contains deprecations (e.g. mappings) a reindex solves them.
+Upgrade Assistant contains migration scripts that are executed as part of the reindex process.
+The user will see a "Reindex" button they can click which will apply this script and perform the
+reindex.
+  * Reindexing is an atomic process in Upgrade Assistant, so that ingestion is never disrupted.
+    It works like this:
+    * Create a new index with a "reindexed-" prefix ([#30114](https://github.com/elastic/kibana/pull/30114)).
+    * Create an index alias pointing from the original index name to the prefixed index name.
+    * Reindex from the original index into the prefixed index.
+    * Delete the old index and rename the prefixed index.
+  * Some apps might require custom scripts, as was the case with APM ([#29845](https://github.com/elastic/kibana/pull/29845)).
+    In that case the migration performed a reindex with a Painless script (covered by automated tests)
+    that made the required changes to the data.
+* **Update index settings.** Some index settings will need to be updated, which doesn't require a
+reindex. An example of this is the "Fix" button that was added for metricbeat and filebeat indices
+([#32829](https://github.com/elastic/kibana/pull/32829), [#33439](https://github.com/elastic/kibana/pull/33439)).
+* **Following the docs.** The Deprecation Info API provides links to the deprecation docs. Users
+will follow these docs to address the problem and make these warnings or errors disappear in the
+Upgrade Assistant.
+* **Stopping/restarting tasks and jobs.** Users had to stop watches and ML jobs and restart them as
+soon as reindexing was complete ([#29663](https://github.com/elastic/kibana/pull/29663)).

x-pack/plugins/watcher/README.md

@@ -1,4 +1,4 @@
-# Conventions
+# Watcher
 
 This plugins adopts some conventions in addition to or in place of conventions in Kibana (at the time of the plugin's creation):
 
@@ -69,4 +69,4 @@ encapsulating operations around such relationships — for example, updating the
 
 ### Kibana client code
 This layer deals almost exclusively with data in the form of client models. The one exception to this rule is when the client code needs
-to bootstrap a model instance from a bare JS object — for example, creating a new `Watch` model from the contents of the Add/Edit Watch Form.
+to bootstrap a model instance from a bare JS object — for example, creating a new `Watch` model from the contents of the Add/Edit Watch Form.