Restructure docs menu pages (#47797)

Docusaurus [sidebar
generation](https://docusaurus.io/docs/next/sidebar/autogenerated)
expects category index pages to have one of three file path conventions:

- `section/index.mdx`
- `section/README.mdx`
- `section/section.mdx`

This change standardizes category index paths on the third convention so
Docusaurus sidebar generation succeeds. We can then add checks to the
current docs site to prevent additional menu pages from violating this
convention.

This change also adds redirects to the new category index pages, and
updates internal links to pages that were moved.

Note that this change does not move all relevant menu pages. We still
need to reorganize the `reference/terraform-provider` section. Since
this section is automatically generated, we need another approach to
restructuring it.

CHANGELOG.md

@@ -386,7 +386,7 @@ applications in Kubernetes clusters. When connected to a Kubernetes cluster (or
 deployed as a Helm chart), the Teleport Discovery Service will automatically find
 and enroll web applications with your Teleport cluster.
 
-See documentation [here](docs/pages/enroll-resources/auto-discovery/kubernetes-applications.mdx).
+See documentation [here](docs/pages/enroll-resources/auto-discovery/kubernetes-applications/kubernetes-applications.mdx).
 
 #### Extended Kubernetes per-resource RBAC
 
@@ -1909,7 +1909,7 @@ is more than one major version behind them. You can use the `--skip-version-chec
 bypass the version check.
 
 Take a look at component compatibility guarantees in the
-[documentation](docs/pages/upgrading.mdx).
+[documentation](docs/pages/upgrading/upgrading.mdx).
 
 #### HTTP_PROXY for reverse tunnels
 
@@ -2898,7 +2898,7 @@ if err = clt.CreateAccessRequest(ctx, accessRequest); err != nil {
 
 ### Upgrade Notes
 
-Please follow our [standard upgrade procedure](docs/pages/admin-guides/management/admin.mdx) to upgrade your cluster.
+Please follow our [standard upgrade procedure](docs/pages/admin-guides/management/admin/admin.mdx) to upgrade your cluster.
 
 Note, for clusters using GitHub SSO and Trusted Clusters, when upgrading SSO users will lose connectivity to leaf clusters. Local users will not be affected.
 
@@ -3149,7 +3149,7 @@ Other updates:
 
 * We now provide local user management via `https://[cluster-url]/web/users`, providing the ability to edit, reset and delete local users.
 * Teleport Node & App Install scripts. This is currently an Enterprise-only feature that provides customers with an 'auto-magic' installer script. Enterprise customers can enable this feature by modifying the 'token' resource. See note above.
-* We've added a Waiting Room for customers using Access Workflows. [Docs](docs/pages/admin-guides/access-controls/access-request-plugins.mdx)
+* We've added a Waiting Room for customers using Access Workflows. [Docs](docs/pages/admin-guides/access-controls/access-request-plugins/access-request-plugins.mdx)
 
 ##### Signed RPM and Releases
 
@@ -3183,7 +3183,7 @@ We've added an [API Guide](docs/pages/admin-guides/api/api.mdx) to simply develo
 
 #### Upgrade Notes
 
-Please follow our [standard upgrade procedure](./docs/pages/upgrading.mdx).
+Please follow our [standard upgrade procedure](docs/pages/upgrading/upgrading.mdx).
 
 * Optional: Consider updating `https_key_file` & `https_cert_file` to our new `https_keypairs:` format.
 * Optional: Consider migrating Kubernetes access from `proxy_service` to `kubernetes_service` after the upgrade.
@@ -3327,7 +3327,7 @@ auth_service:
 #### Upgrade Notes
 
 Please follow our [standard upgrade
-procedure](docs/pages/upgrading.mdx).
+procedure](docs/pages/upgrading/upgrading.mdx).
 
 ## 4.3.9
 
@@ -3412,7 +3412,7 @@ Teleport's Web UI now exposes Teleport’s Audit log, letting auditors and admin
 
 ##### Teleport Plugins
 
-Teleport 4.3 introduces four new plugins that work out of the box with [Approval Workflow](docs/pages/admin-guides/access-controls/access-request-plugins.mdx). These plugins allow you to automatically support role escalation with commonly used third party services. The built-in plugins are listed below.
+Teleport 4.3 introduces four new plugins that work out of the box with [Approval Workflow](docs/pages/admin-guides/access-controls/access-request-plugins/access-request-plugins.mdx). These plugins allow you to automatically support role escalation with commonly used third party services. The built-in plugins are listed below.
 
 *   [PagerDuty](docs/pages/admin-guides/access-controls/access-request-plugins/ssh-approval-pagerduty.mdx)
 *   [Jira](docs/pages/admin-guides/access-controls/access-request-plugins/ssh-approval-jira.mdx)
@@ -3448,7 +3448,7 @@ Teleport 4.3 introduces four new plugins that work out of the box with [Approval
 #### Upgrade Notes
 
 Always follow the [recommended upgrade
-procedure](./docs/pages/upgrading.mdx) to upgrade to this version.
+procedure](docs/pages/upgrading/upgrading.mdx) to upgrade to this version.
 
 ##### New Signing Algorithm
 
@@ -3489,7 +3489,7 @@ permissions](./docs/pages/enroll-resources/kubernetes-access/controls.mdx).
 The [etcd backend](docs/pages/reference/backends.mdx#etcd) now correctly uses
 the “prefix” config value when storing data. Upgrading from 4.2 to 4.3 will
 migrate the data as needed at startup. Make sure you follow our Teleport
-[upgrade guidance](docs/pages/upgrading.mdx).
+[upgrade guidance](docs/pages/upgrading/upgrading.mdx).
 
 **Note: If you use an etcd backend with a non-default prefix and need to downgrade from 4.3 to 4.2, you should [backup Teleport data and restore it](docs/pages/admin-guides/management/operations/backup-restore.mdx) into the downgraded cluster.**
 
@@ -3612,7 +3612,7 @@ This is a minor Teleport release with a focus on new features and bug fixes.
 ### Improvements
 
 * Alpha: Enhanced Session Recording lets you know what's really happening during a Teleport Session. [#2948](https://github.com/gravitational/teleport/issues/2948)
-* Alpha: Workflows API lets admins escalate RBAC roles in response to user requests. [Read the docs](docs/pages/admin-guides/access-controls/access-requests.mdx). [#3006](https://github.com/gravitational/teleport/issues/3006)
+* Alpha: Workflows API lets admins escalate RBAC roles in response to user requests. [Read the docs](docs/pages/admin-guides/access-controls/access-requests/access-requests.mdx). [#3006](https://github.com/gravitational/teleport/issues/3006)
 * Beta: Teleport provides HA Support using Firestore and Google Cloud Storage using Google Cloud Platform. [Read the docs](docs/pages/admin-guides/deploy-a-cluster/deployments/gcp.mdx). [#2821](https://github.com/gravitational/teleport/pull/2821)
 * Remote tctl execution is now possible. [Read the docs](./docs/pages/reference/cli/tctl.mdx). [#1525](https://github.com/gravitational/teleport/issues/1525) [#2991](https://github.com/gravitational/teleport/issues/2991)
 
@@ -3868,7 +3868,7 @@ The lists of improvements and bug fixes above mention only the significant chang
 
 ### Upgrading
 
-Teleport 4.0 is backwards compatible with Teleport 3.2 and later. [Follow the recommended upgrade procedure to upgrade to this version.](docs/pages/upgrading.mdx)
+Teleport 4.0 is backwards compatible with Teleport 3.2 and later. [Follow the recommended upgrade procedure to upgrade to this version.](docs/pages/upgrading/upgrading.mdx)
 
 Note that due to substantial changes between Teleport 3.2 and 4.0, we recommend creating a backup of the backend datastore (DynamoDB, etcd, or dir) before upgrading a cluster to Teleport 4.0 to allow downgrades.
 
@@ -4136,7 +4136,7 @@ on Github for more.
 #### Upgrading to 3.0
 
 Follow the [recommended upgrade
-procedure](docs/pages/upgrading.mdx) to upgrade to this
+procedure](docs/pages/upgrading/upgrading.mdx) to upgrade to this
 version.
 
 **WARNING:** if you are using Teleport with the etcd back-end, make sure your
@@ -4242,7 +4242,7 @@ As always, this release contains several bug fixes. The full list can be seen [h
 #### Upgrading
 
 Follow the [recommended upgrade
-procedure](docs/pages/upgrading.mdx) to upgrade to this
+procedure](docs/pages/upgrading/upgrading.mdx) to upgrade to this
 version.
 
 ## 2.6.9
@@ -4372,7 +4372,7 @@ You can see the full list of 2.6.0 changes [here](https://github.com/gravitation
 #### Upgrading
 
 Follow the [recommended upgrade
-procedure](docs/pages/upgrading.mdx) to upgrade to this
+procedure](docs/pages/upgrading/upgrading.mdx) to upgrade to this
 version.
 
 ## 2.5.7
@@ -4459,7 +4459,7 @@ release, which includes:
 
 * The Teleport daemon now implements built-in connection draining which allows
   zero-downtime upgrades.  [See
-  documentation](docs/pages/upgrading.mdx).
+  documentation](docs/pages/upgrading/upgrading.mdx).
 
 * Dynamic join tokens for new nodes can now be explicitly set via `tctl node add --token`.
   This allows Teleport admins to use an external mechanism for generating