Restore Pages deployment and keep fork Actions clean

[koo-ec]

What changed

• Build and validate documentation, upload it as a Pages artifact, and deploy with GitHub's supported deploy-pages action instead of only pushing gh-pages.
• Limit documentation deployment, PyPI publishing, and PageSpeed audits to the upstream repository. The production PageSpeed audit also skips pull requests.
• Keep fork CI tests, while skipping Codecov uploads where the upstream token is unavailable.
• Serialize documentation runs to avoid cancelling a deployment midway.

Cause and resolution

GitHub Pages had Deploy from a branch selected with the branch set to None, so the site returned 404 although gh-pages contained index.html. I switched the repository's Pages source to GitHub Actions, matching the new workflow.

Verification

• All four workflow YAML files parsed; git diff --check passed.
• The final PR commit passed Documentation and Continuous Integration checks; PageSpeed Audit correctly skipped the PR.
• The repository now reports has_pages: true. The live URL will be checked after merge and deployment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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