Proposal: Clean up the Docs menu and navigation

Currently, the Harbor docs menu is unweildy; while missing the most recent version, it contains 16 unique minor versions. I propose the following cleanup steps:

- Replace all 'pretty old' versions with a 'Legacy' link. 2.10.3, released Jul 2024, is the last 2.10 release. It seems like a reasonable starting point.
- Add an more complete 'versions' dropdown, JS-based, to a documentation template where the version indicator is by the Harbor logo. This allows jumping between all versions of a specific page. For example, if I had a 2.5 install languishing somewhere, being able to jump between the 'Upgrade' pages to figure out my upgrade chain would be useful. There may be 404s from this; update the 404 page to mention this (or manage checking versions for each page, but that seems like a substantial effort).
- In the menu, instead of calling them 2.7.0, etc., call them 2.7.x or 2.7. The doc notes, from what I can tell, don't change in minor versions in a breaking way. The 2.7.x nomenclature is used at the top of the root 2.7 doc page, so this seems reasonable.
- Update the URL convention and have redirects for the existing versions.