Refresh README for v3 + add v3 branch-preview link

Details

- add a "Live & preview" block: production (krichelj.github.io, main only) and
  the free Netlify v3 branch-deploy URL for quick access from the v3 branch
- document the three coexisting site versions (v3 / v2 / v1) + routing and the
  v3 feature highlights
- point to PREVIEW.md for branch-preview options; note .ruby-version (3.2) and
  the no-secrets local-serve path (local.sh runs the GA4 dashboard)
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HShRpQUmDUttxD5F89ziDu

Files changed (1)