Add basic docs for the GitLab CLI

What does this MR do and why?

Describe in detail what your merge request does and why.

This stubs out docs for the GitLab CLI so that we at least have some reference to it.

Related issues

• Closes Add GitLab CLI to main docs site (cli#1101 - closed)
• Related to Determine how to handle version history for out... (#382992 - closed) (follow-up to sort out how to handle version history for outside projects)

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