Plant anchors for hunks to create links in doc preview changes

[kwankyu]

The "changes" page of the doc preview of a PR lacks links to the actual changed parts of the doc preview. We plant invisible anchors for each hunk of the diffs and use them to create links to the changed parts. A sample page is

https://deploy-preview-36742--sagemath-tobias.netlify.app/changes

Wishlist item: In the diffsite comparison view, the left pane showing the doc preview is scrolled to the changed part while the right pane showing the base doc does not. It would be nice if the right pane shows the corresponding part. But this cannot be implemented now because the base doc cannot be altered for specific doc preview for a PR.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[mkoeppe]

At some point it may be a good idea to split out this diff script to a separate file (perhaps in .ci/) so that it can be run and tested outside of the GH Actions workflow

[kwankyu]

At some point it may be a good idea to split out this diff script to a separate file (perhaps in .ci/) so that it can be run and tested outside of the GH Actions workflow

Done. Thanks for the idea.

[github-actions]

Documentation preview for this PR (built with commit 7b0c670; changes) is ready! 🎉

[kwankyu]

python3 works.

[mkoeppe]

LGTM!

[kwankyu]

Thanks a lot!