[3.9] [doc] bpo-45680: Disambiguate ``__getitem__`` and ``__class_getitem__`` in the data model (GH-29389) (GH-29619)

The documentation explaining Python's data model does not adequately explain
the differences between ``__getitem__`` and ``__class_getitem__``, nor does it
explain when each is called. There is an attempt at explaining
``__class_getitem__`` in the documentation for ``GenericAlias`` objects, but
this does not give sufficient clarity into how the method works. Moreover, it
is the wrong place for that information to be found; the explanation of
``__class_getitem__`` should be in the documentation explaining the data model.

This PR has been split off from GH-29335.
(cherry picked from commit 31b3a70edb1216bdc8fab3b2eafd8ddb00487f41)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

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