docs: add doc version for v1.12 and v2.5 fixes (#676)

## Summary

- Add versioned docs for v1.12 (latest 1.x) from git tag `v1.12.2`.
- Fix Python version requirement in v2.5 docs (3.11+ → 3.10+).
- Override `DocsVersionDropdownNavbarItem` so the version dropdown
navigates to `reference/<version>` instead of `docs/<version>` when on
API reference pages.
  - Same as in the JS version.

## Known limitation

- SubNavbar links (Docs/Reference/Changelog) don't preserve the selected
version — tracked in #675.
  - This applies to JS repos (both client & SDK) as well.

## Test plan

- [x] Tested locally
- [x] CI passes