[3.13] gh-136672: Docs: Move parts of Enum HOWTO to API Docs (GH-139176) (GH-144803)

To avoid duplicate content in the Enum HOWTO and
API documentation which is not automatically synced,
the section about supported __dunder__ and _sunder
names is moved from HOWTO to API docs.

See also https://github.com/python/cpython/pull/136791
(cherry picked from commit 629a363ddd2889f023d5925506e61f5b6647accd)

Co-authored-by: Rafael Weingartner-Ortner <38643099+RafaelWO@users.noreply.github.com>