[3.13] gh-124720: Update "Using Python on a Mac" document (GH-124721) (#124775)

gh-124720: Update "Using Python on a Mac" document (GH-124721)

Update "Using Python on a Mac" section of the "Python Setup and Usage"
document and include information on installing free-threading support.
(cherry picked from commit 4b83c03ce964af7fb204144db9adaa524c113a82)

Co-authored-by: Ned Deily <nad@python.org>