Updates navigation titles and descriptions for release notes (#215422)

## Summary

Updates the navigation titles, adds short descriptions, and removes
release dates.

docs/release-notes/breaking-changes.md

@@ -1,16 +1,15 @@
 ---
-navigation_title: "Kibana"
+navigation_title: "Breaking changes"
 mapped_pages:
   - https://www.elastic.co/guide/en/kibana/current/breaking-changes-summary.html
 ---
 
 # Kibana breaking changes [kibana-breaking-changes]
-Before you upgrade, carefully review the Kibana breaking changes and take the necessary steps to mitigate any issues. 
+Breaking changes can impact your Elastic applications, potentially disrupting normal operations. Before you upgrade, carefully review the Kibana breaking changes and take the necessary steps to mitigate any issues.
 
-If you are migrating from a version prior to version 9.0, you must first upgrade to the last 8.x version available. To learn how to upgrade, check out <uprade docs>.
+If you are migrating from a version prior to version 9.0, you must first upgrade to the last 8.x version available. To learn how to upgrade, check out [Upgrade](docs-content://deploy-manage/upgrade.md).
 
 % ## Next version [kibana-nextversion-breaking-changes]
-% **Release date:** Month day, year
 
 % Use the following template to add entries to this document.
 
@@ -38,7 +37,6 @@ If you are migrating from a version prior to version 9.0, you must first upgrade
 % Check https://docs.elastic.dev/docs/kibana-doc-links (internal) for more details about the Doc links service.
 
 ## 9.0.0 [kibana-900-breaking-changes]
-**Release date:** March 25, 2025
 
 ::::{dropdown} Removed legacy alerting endpoints
 * `POST /api/alerts/alert/{id?}` has been replaced by `POST /api/alerting/rule/{id?}`

docs/release-notes/deprecations.md

@@ -1,14 +1,13 @@
 ---
-navigation_title: "Kibana"
+navigation_title: "Deprecations"
 ---
 
 # Kibana deprecations [kibana-deprecations]
-Review the deprecated functionality for your Kibana version. While deprecations have no immediate impact, we strongly encourage you update your implementation after you upgrade.
+Over time, certain Elastic functionality becomes outdated and is replaced or removed. To help with the transition, Elastic deprecates functionality for a period before removal, giving you time to update your applications.
 
-To learn how to upgrade, check out <uprade docs>.
+Review the deprecated functionality for Kibana. While deprecations have no immediate impact, we strongly encourage you update your implementation after you upgrade. To learn how to upgrade, check out [Upgrade](docs-content://deploy-manage/upgrade.md).
 
 % ## Next version [kibana-versionnext-deprecations]
-% **Release date:** Month day, year
 
 % Use the following template to add entries to this document.
 
@@ -36,7 +35,6 @@ To learn how to upgrade, check out <uprade docs>.
 % Check https://docs.elastic.dev/docs/kibana-doc-links (internal) for more details about the Doc links service.
 
 ## 9.0.0 [kibana-900-deprecations]
-**Release date:** March 25, 2025
 
 ::::{dropdown} Scripted field creation has been disabled in the Data Views management page
 The ability to create new scripted fields has been removed from the **Data Views** management page in 9.0. Existing scripted fields can still be edited or deleted, and the creation UI can be accessed by navigating directly to `/app/management/kibana/dataViews/dataView/{{dataViewId}}/create-field`, but we recommend migrating to runtime fields or ES|QL queries instead to prepare for removal.

docs/release-notes/index.md

@@ -15,7 +15,6 @@ To check for security updates, go to [Security announcements for the Elastic sta
 % Release notes include only features, enhancements, and fixes. Add breaking changes, deprecations, and known issues to the applicable release notes sections. 
 
 % ## version.next [kibana-next-release-notes]
-% **Release date:** Month day, year
 
 % ### Features and enhancements [kibana-next-features-enhancements]
 % * 
@@ -24,7 +23,6 @@ To check for security updates, go to [Security announcements for the Elastic sta
 % * 
 
 ## 9.0.0 [kibana-900-release-notes]
-**Release date:** March 25, 2025
 
 ### Features and enhancements [kibana-900-features-enhancements]
 

docs/release-notes/known-issues.md

@@ -1,5 +1,5 @@
 ---
-navigation_title: "Kibana"
+navigation_title: "Known issues"
 ---
 
 # Kibana known issues