Python "HOWTO" guide for free-threading  #124370

We already have a "HOWTO" guide for free-threading C API extension authors.

We should have a similar guide aimed at people writing Python code. The HOWTO should document the current limitations of the free-threaded build.

Linked PRs

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