[8.11] Enhance plugin documentation (#146678) (#172104)

# Backport

This will backport the following commits from `main` to `8.11`:
- [Enhance plugin documentation
(#146678)](https://github.com/elastic/kibana/pull/146678)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

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improvements for the new `'notifications'`\nplugin documentation:\n\n-
Add a link to the plugin API description in the kibana-dev-docs
nav\nbar.\n- Convert the README to
`mdx`.\n\n---------\n\nCo-authored-by: kibanamachine
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improvements for the new `'notifications'`\nplugin documentation:\n\n-
Add a link to the plugin API description in the kibana-dev-docs
nav\nbar.\n- Convert the README to
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improvements for the new `'notifications'`\nplugin documentation:\n\n-
Add a link to the plugin API description in the kibana-dev-docs
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BACKPORT-->

Co-authored-by: Gerard Soldevila <gerard.soldevila@elastic.co>

docs/developer/plugin-list.asciidoc

@@ -668,7 +668,7 @@ Elastic.
 |This plugin allows for other plugins to add data to Kibana stack monitoring documents.
 
 
-|{kib-repo}blob/{branch}/x-pack/plugins/notifications/README.md[notifications]
+|{kib-repo}blob/{branch}/x-pack/plugins/notifications/README.mdx[notifications]
 |The Notifications plugin provides a set of services to help Solutions and plugins send notifications to users.
 
 

docs/setup/settings.asciidoc

@@ -323,8 +323,7 @@ run {kib} in different modes. Valid options are `background_tasks` and `ui`,
 or `*` to select all roles. *Default: `*`*
 
 `notifications.connectors.default.email`::
-Specifies the name of the connector that is used to send email notifications.
-In {ecloud}, the default value is `elastic-cloud-email`.
+ Choose the default email connector for user notifications. As of `8.6.0`, {kib} is shipping with a new notification mechanism that will send email notifications for various user actions, e.g. assigning a _Case_ to a user. To enable notifications, an email connector must be <<pre-configured-connectors,preconfigured>> in the system via `kibana.yml`, and the notifications plugin must be configured to point to the ID of that connector.
 
 [[path-data]] `path.data`::
 The path where {kib} stores persistent data

nav-kibana-dev.docnav.json

@@ -411,6 +411,9 @@
         {
           "id": "kibNewsfeedPluginApi"
         },
+        {
+          "id": "kibNotificationsPluginApi"
+        },
         {
           "id": "kibObservabilityPluginApi"
         },