Fix docs issues that break Docusaurus builds (#47491)

Backports #47471

- Add index pages to top-level docs sections. These pages do not appear
  in the sidebar of the current docs engine, but are required for the
  Docusaurus site to render properly. We can build them into fully
  developed introductory pages in subsequent changes.

docs/pages/admin-guides/admin-guides.mdx

@@ -0,0 +1,6 @@
+---
+title: Teleport Admin Guides
+description: Provides step-by-step instructions for completing administrative tasks in Teleport.
+---
+
+(!toc!)

docs/pages/connect-your-client/connect-your-client.mdx

@@ -0,0 +1,6 @@
+---
+title: Teleport User Guides
+description: Provides instructions to help users connect to infrastructure resources with Teleport.
+---
+
+(!toc!)

docs/pages/enroll-resources/enroll-resources.mdx

@@ -0,0 +1,6 @@
+---
+title: Enrolling Teleport Resources
+description: Provides step-by-step instructions for enrolling servers, databases, and other infrastructure resources with your Teleport cluster.
+---
+
+(!toc!)

docs/pages/reference/reference.mdx

@@ -0,0 +1,6 @@
+---
+title: Teleport Reference Guides
+description: Provides comprehensive information on configuration fields, Teleport commands, and other ways of interacting with Teleport.
+---
+
+(!toc!)