bpo-37860: Enable docs deploy previews through GitHub Actions

[epicfaace]

Sample PR: epicfaace#7

https://bugs.python.org/issue37860

[hugovk]

This looks good, although one downside is it creates a new directory for each and every commit pushed to a PR:

• https://github.com/epicfaace/cpython/tree/gh-pages
• 586 MB for 10 builds

That's going to get pretty big and fast, so would need to have some way to cleanup old ones.

It would be better to use the PR number instead of commit SHA and only build PRs that change docs or docs build logic.

Also I wonder if that amount of churn on a single branch would have a performance impact on the rest of the Git repo?

Perhaps another host server would be better.

[epicfaace]

• 586 MB for 10 builds

Yeah, and GitHub Pages also has a hard limit of 1 GB in size -- https://docs.github.com/en/pages/getting-started-with-github-pages/about-github-pages#usage-limits. Why is the docs site so large again? Are there certain large static files that can be symlinked / shared between deploy previews?

It would be better to use the PR number instead of commit SHA and only build PRs that change docs or docs build logic.

Good idea.

Also I wonder if that amount of churn on a single branch would have a performance impact on the rest of the Git repo?

I don't think it will be an issue as long as we clean up merged PRs -- so we only include PRs in this branch that are currently open.

[epicfaace]

Closing in favor of #92852