Update all documentation links from `modelcontextprotocol.github.io/ext-apps`
to the new custom domain across README.md, docs/, and example READMEs.
Add `cname` to the `peaceiris/actions-gh-pages` deploy action in
`.github/workflows/docs.yml` so the CNAME file persists across deploys.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>