bpo-39452: Rewrite and expand __main__.rst (#26883)

Broadened scope of the document to explicitly discuss and differentiate between ``__main__.py`` in packages versus the ``__name__ == '__main__'`` expression (and the idioms that surround it), as well as ``import __main__``.

Co-authored-by: Géry Ogam <gery.ogam@gmail.com>
Co-authored-by: Éric Araujo <merwok@netwok.org>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>