[3.13] gh-119241: Add HOWTO for free-threaded C API extensions (GH-119877) (#120693)

Some sections adapted from https://github.com/Quansight-Labs/free-threaded-compatibility/
written by Nathan Goldbaum.

(cherry picked from commit 02b272b7026b68e70b4a4d9a0ca080904aed374c)

Co-authored-by: Sam Gross <colesbury@gmail.com>
Co-authored-by: Nathan Goldbaum <nathan.goldbaum@gmail.com>