nav: Add Repository consistency checks subpage

What does this MR do and why?

As part of gitlab#437663 (closed), adds a navbar entry for the new Repository consistency checks subpage in the Gitaly doco.

Note that gitlab!143463 (merged) must be merged first, as that MR actually adds the subpage.

Screenshots, screen recordings, or links to review app

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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.