Docs maintenance: Add pages to left nav

What does this MR do and why?

• Adds pages to the left nav as part of the docs project maintenance: #1656 (closed)
• Ensures all entries listed under https://docs.gitlab.com/ee/administration/auth/ are consistently alphabetized

Screenshots, screen recordings, or links to review app

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.