Promote Git content on global navigation

What does this MR do and why?

For: https://gitlab.com/gitlab-org/gitlab/-/issues/425355, improve the visibility of our Git documentation by moving it up to the top-level on the global navigation.

Screenshots, screen recordings, or links to review app

Preview: https://eread-combine-and-promote-git-content-on-global-navigation.docs.gitlab-review.app/ee/topics/git/

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.