New menu item for Gitaly Cluster troubleshooting information

What does this MR do and why?

Because of a page split in gitlab!141956 (merged), add new page to global navigation.

Screenshots, screen recordings, or links to review app

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

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.