Migrate docs hosting from Read the Docs to GitHub Pages

[lesnik512]

Summary

• Adds .github/workflows/docs.yml mirroring modern-di's pattern: mkdocs gh-deploy --force on push to main (paths-filtered), concurrency-serialized, contents: write.
• Adds just docs-deploy target driving the deploy via uvx --with-requirements docs/requirements.txt mkdocs gh-deploy --force.
• Adds docs/CNAME and mkdocs.yml site_url for the custom domain faststream-outbox.modern-python.org.
• Deletes .readthedocs.yaml; updates the four readthedocs.io links in README.md to modern-python.org equivalents.
• Bumps action pins in ci.yml and publish.yml to the same baseline the new docs.yml uses (checkout@v6, setup-just@v4, setup-uv@v8.2.0).

Spec: planning/specs/2026-06-09-mkdocs-github-pages-design.md

Out-of-repo steps after merge

1. DNS: CNAME faststream-outbox.modern-python.org → modern-python.github.io.
2. After the first workflow run creates gh-pages, set Settings → Pages → Source = gh-pages branch, / root.
3. Tick "Enforce HTTPS" once the cert provisions (~5 min).
4. Archive the faststream-outbox project on readthedocs.io.

Test plan

• PR CI is green (ci.yml lint + pytest jobs run with bumped pins).
• After merge, the Deploy Docs workflow runs and creates the gh-pages branch.
• https://faststream-outbox.modern-python.org/ returns the docs site once DNS + Pages settings are in place.
• All four README.md links resolve to working pages on the new domain.
• A subsequent push to main touching docs/ re-deploys.

🤖 Generated with Claude Code