[3.12] gh-101100: Only show GitHub check annotations on changed doc paragraphs (GH-108065) (#108127)

gh-101100: Only show GitHub check annotations on changed doc paragraphs (GH-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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(cherry picked from commit eb953d6e4484339067837020f77eecac61f8d4f8)

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