gh-101100: Only show GitHub check annotations on changed doc paragraphs (#108065)

* Only show GitHub check annotations on changed doc paragraphs
* Improve check-warnings script arg parsing following Hugo's suggestions
* Factor filtering warnings by modified diffs into helper function
* Build docs on unmerged branch so warning lines match & avoid deep clone

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Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>