Add missing left nav for commits

Relates to Monthly docs chores, Pages not in the nav Dec 2022 (#1329 - closed)

The Commits tab in merge requests page is not in the left nav.

Related MRs

• Split, shift, recombine basic MR information (gitlab!62424 - merged) - splits the content into its own page
• Add new pages about merge request tabs (!1840 - closed) - adds the global nav item, closed without merging. Some discussion around larger changes.