Co-authored-by: Copilot <copilot@github.com>

…penapi.org

Adds the publishing-pipeline pieces required by the unified
python-openapi.org website shell:

- docs/overrides/main.html: MkDocs theme override that injects the
  shared ecosystem banner and footer per docs-theme-contract.md
- mkdocs.yml: theme.custom_dir wired to docs/overrides
- docs/manifest.json: the manifest the shell consumes to resolve
  versions and the Pagefind bundle URL
- Makefile: docs-pagefind, docs-manifest, and docs-publish targets
  used by the docs publishing pipeline
- .readthedocs.yaml: post_build hook that runs `make docs-publish`
  and copies docs_build/* into the RTD output, so each published
  build also exposes /pagefind/ and /manifest.json next to the HTML

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>