Add GitHub Actions workflow to publish API docs to gh-pages on master push

[Copilot]

API docs need to be automatically rebuilt and published to the gh-pages branch whenever master is updated.

Changes

• .github/workflows/docs.yml — new workflow triggered on master push that:
  • Installs DocFX via dotnet tool install -g docfx
  • Builds the static site from docs/docfx.json → docs/_site/
  • Deploys docs/_site/ to gh-pages using peaceiris/actions-gh-pages@v4 with the built-in GITHUB_TOKEN (contents: write permission scoped to the job)

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[jonathanvdc]

LGTM