gitlab-docs: Update Git undo link in left nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!146271 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Updates the left nav to reflect this page move:

• Old: doc/topics/git/numerous_undo_possibilities_in_git/index.md
• New: doc/topics/git/undo.md

Related links

• Related to Create IA work: fix the 'undo' page (gitlab#443858 - closed)
• Blocked until Make Git 'undo' page URL much shorter (gitlab!146271 - merged) is merged.