Merge branch 'docs/add-admin-area-index' into 'master'

Add an index page to admin area section

Closes #55234

See merge request gitlab-org/gitlab-ce!25375

doc/administration/index.md

@@ -53,6 +53,7 @@ Learn how to install, configure, update, and maintain your GitLab instance.
 - [Diff limits](../user/admin_area/diff_limits.md): Configure the diff rendering size limits of branch comparison pages.
 - [Merge request diffs storage](merge_request_diffs.md): Configure merge requests diffs external storage.
 - [Broadcast Messages](../user/admin_area/broadcast_messages.md): Send messages to GitLab users through the UI.
+- [Admin Area](../user/admin_area/index.md): for self-managed instance-wide configuration and maintenance.
 
 #### Customizing GitLab's appearance
 

doc/user/admin_area/index.md

+# GitLab Admin Area **[CORE ONLY]**
+
+The Admin Area provides a web UI for administering some features of GitLab self-managed instances.
+
+To access the Admin Area, either:
+
+- Click the Admin Area icon (the spanner or wrench icon).
+- Visit `/admin` on your self-managed instance.
+
+NOTE: **Note:**
+Only admin users can access the Admin Area.
+
+## Admin Area sections
+
+The Admin Area is made up of the following sections:
+
+| Section           | Description                                                                                                                                              |
+|:------------------|:---------------------------------------------------------------------------------------------------------------------------------------------------------|
+| Overview          | View your GitLab Dashboard, and maintain projects, users, groups, jobs, runners, and Gitaly servers.                                                     |
+| Monitoring        | View GitLab system information, and information on background jobs, logs, [health checks](monitoring/health_check.md), request profiles, and audit logs. |
+| Messages          | Send and manage [broadcast messages](broadcast_messages.md) for your users.                                                                              |
+| System Hooks      | Configure [system hooks](../../system_hooks/system_hooks.md) for many events.                                                                            |
+| Applications      | Create system [OAuth applications](../../integration/oauth_provider.md) for integrations with other services.                                            |
+| Abuse Reports     | Manage [abuse reports](abuse_reports.md) submitted by your users.                                                                                        |
+| Deploy Keys       | Create instance-wide [SSH deploy keys](../../ssh/README.md#deploy-keys).                                                                                 |
+| Service Templates | Create [service templates](../project/integrations/services_templates.md) for projects.                                                                   |
+| Labels            | Create and maintain [labels](labels.md) for your GitLab instance.                                                                                        |
+| Appearance        | Customize [GitLab's appearance](../../customization/index.md).                                                                                           |
+| Settings          | Modify the [settings](settings/index.md) for your GitLab instance.                                                                                       |