Backport Versions menu to 15.1

What does this MR do and why

Backports the Versions menu to 15.1 (#1238 (closed)).

How to test

1. View the site in the Review App: http://1238-backport-versions-menu-151.35.193.151.162.nip.io/r
2. Verify the Versions menu shows the same Versions as the main branch, docs.gitlab.com
3. Verify the menu displays correctly at large and small widths
4. Check for errors in the browser console

Caveats for the Review app:

• The menu will default to 15.2 in the Review app because the Review app does not deploy older versions with the same URLs as production (e.g, docs.gitlab.com/15.1/whatever)
• Links to other versions will not work in the Review app