gh-124370: Add "howto" for free-threaded Python (#124371)

* gh-124370: Add "howto" for free-threaded Python

This is a guide aimed at people writing Python code, as oppposed to the
existing guide for C API extension authors.

* Add missing new line

* Update Doc/howto/free-threading-python.rst

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

* interned -> immortalized

* Apply suggestions from code review

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

* Update Doc/howto/free-threading-python.rst

Co-authored-by: mpage <mpage@cs.stanford.edu>

* Update docs

* Apply suggestions from code review

Co-authored-by: Carol Willing <carolcode@willingconsulting.com>

* A few more updates

* Additional comment on immortal objects

* Mention specializing adaptive interpreter

* Remove trailing whitespace

* Remove mention of C macro

---------

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>