gh-36742: Plant anchors for hunks to create links in doc preview changes
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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.
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/36742
Reported by: Kwankyu Lee
Reviewer(s): Kwankyu Lee, Matthias Köppe
