C API Extension Support for Freethreading HOWTO

[colesbury]

We should add a "HOWTO" guide for C API extension authors covering how to support the free-threaded build.

See also https://peps.python.org/pep-0703/#how-to-teach-this

Linked PRs

• gh-119241: Add HOWTO for free-threaded C API extensions #119877
• [3.13] gh-119241: Add HOWTO for free-threaded C API extensions (GH-119877) #120693
• gh-124370: Add "howto" for free-threaded Python #124368