Add nav item for new reduce repository size page

Add navigation item to global navigation

• Link to the merge request that introduced the new page: Add new reduce repository size page (gitlab!170943 - merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://bl-nav-item-for-new-page.docs.gitlab-review.app/ee/topics/git/repository.html

Navigation

- Advanced operations
   - Rebase and resolve conflicts
   - Revert and undo changes
   - Reduce repository size
   - Tutorial: Update Git remote URLs

Related issues

• Issue: gitlab#499459
• Child item: gitlab#499467