Configure GitHub Pages for HTML docs previews

[jdillard]

Summary of changes

• Configure GitHub Pages for HTML docs previews
  • I configured my fork and you can see the docs preview for that PR as a demo.
    • I'll delete GitHub pages for the fork once this merges so there aren't duplicate docs deployed.
  • I chose GitHub Pages to make ownership of the deployed docs more agnostic since this is in sphinx-contrib, a Read the Docs configuration would be simpler, but more tied to the maintainer.

TODO (after merge)

After merging the workflow, GitHub Pages will need to be configure for the repository:

1. Go to Settings → Pages
2. Under "Build and deployment":
   • Source: Deploy from a branch
   • Branch: gh-pages / / (root)

The workflow will create the gh-pages branch and the .nojekyll file automatically on the first run.

The docs will then be available at:

• Production: https://sphinx-contrib.github.io/mcp/
• PR previews: https://sphinx-contrib.github.io/mcp/pr-preview/pr-<number>/