[3.12] Docs: Run ``latexmk`` in parallel when creating PDFs (GH-123113) (#123116)

Docs: Run ``latexmk`` in parallel when creating PDFs (GH-123113)
(cherry picked from commit 79c542b5cc774ba758acc2b2e3b6556934190e34)

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