sagemathgh-36473: CI docbuild: Do not rebuild sagelib from scratch
    
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Reworking sagemath#35652 as discussed in
sagemath#36469 (comment)

Also applying the changes made in sagemath#35866 (cosmetic improvements to the
Actions logs of doc-build.yml) to doc-build-pdf.yml

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URL: sagemath#36473
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe