Promote v2 to main (/), move classic to /v1/

Details

The v2 layout has been the de-facto main look for a while; serve it at
the canonical root and keep the classic layout reachable for archival.
- Move shared intro content into _includes/intro.md so both layouts can
  pull from one source.
- Repurpose frontend/index.md as the v2 home page (was frontend/v2.md).
- Add frontend/v1.md with permalink: /v1/ for the classic layout.
- Cross-links: classic's pill now reads "New site" and points to /;
  v2's nav + footer "Classic site" links point to /v1/.
- Drop noindex,nofollow from the v2 layout and add a self-referential
  canonical now that this is the indexed version.
- Update load-section.ts so the classic "Introduction" reload fetches
  /v1/index.html (was /index.md, which no longer exists).
Made-with: Cursor

Files changed (9)