[docs] improve how the docs are versioned

[MattSturgeon]

Currently we have two builds of nixvim's docs, one at / and another at /stable/.

There is also no obvious mention of this on either website, so you may not know the other exists if you visit one or the other.

• switch from /stable/ to a versioned url (e.g. /24.11/)
  [Docs] Use lib.trivial.version to segment the docs #2455
• implement redirects from /stable/** to /24.11/**
• link to other docs version in the TOC sidebar
• discuss (and link to) other docs versions on the index page
• Fix "source" link prefix in option-search - it is currently hard-coded to the main branch URL
  See docs/search: refactor to use override #2536 (comment)
• (after branching off nixvim 24.11) maintain three docs websites, 24.05, 24.11, 25.05(unstable)

We should consider whether the unstable docs should live at /, /unstable/ or /25.05/. If not /, we should have redirect files, like for /stable/.