Backport Versions menu to 15.1

This MR adds the new Versions menu to the 15.2 docs site.

This was released and then rolled back, so this MR:

• Reverts this revert: !2931 (merged)
• Adds this fix: !2936 (merged)

How to test

See !2970 (merged).

These are the same changes and can be tested with the same steps, just use 15.1 anywhere 15.2 is referenced.