Add free-threaded to style guide (#1353) (#1722)

* Add 'free-threaded' to style guide (#1353)

* Update documentation/style-guide.rst

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

* Final cleanup: alphabetical order and whitespace

* Apply maintainer feedback: fix style and formatting

* Fix documentation style guide formatting

- Correct CPU indentation to 3 spaces
- Correct free-threaded indentation to 3 spaces
- Change 'prevent' to 'avoid' as suggested by reviewer

* Remove proposed change.

* Add back reference label for style guide

---------

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

Author: 3 people

documentation/style-guide.rst

@@ -79,6 +79,11 @@ C API
 CPU
    Central processing unit. No need to spell out.
 
+free-threaded
+   The preferred term for the build mode that makes the global interpreter
+   lock (GIL) optional (per :pep:`703`). Avoid using "No-GIL" to avoid
+   double negatives (for example, "non-no-GIL").
+
 POSIX
    The name assigned to a particular group of standards. This is always
    uppercase.