[3.13] gh-124370: Add "howto" for free-threaded Python (GH-124371) (#124860)

gh-124370: Add "howto" for free-threaded Python (GH-124371)
(cherry picked from commit 68e384c2179fba41bc3be469e6ef34927a37f4a5)

Co-authored-by: Sam Gross <colesbury@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: mpage <mpage@cs.stanford.edu>
Co-authored-by: Carol Willing <carolcode@willingconsulting.com>