Account for the new tiered support policy

[PaintNinja]

A new support policy was instated that made support for older versions (including non-LTS) permitted and official, with different tiers having varying levels of support: https://forums.minecraftforge.net/topic/144690-new-tiered-support-policy

Current and LTS (currently 1.20.x, 1.20.1, 1.19.x and 1.19.2) fall under the "Full support" tier, older versions under the "Legacy support" tier and short-lived stop-gap versions (such as 1.20.3) falling under the "Minimal support" tier.

Following this change, it would be great if the documentation could be updated accordingly. For instance, the description and "important" disclaimer on this page are now outdated: https://docs.minecraftforge.net/en/latest/legacy/

We could also add all available versions to the navigation dropdown (ideally sorted by newest first instead of oldest first) and change the "note" disclaimer for old versions at the top to mention something along the lines of 'newer versions of the documentation are prioritised so may be helpful to cross-reference'.