bpo-45680: Clarify documentation on ``GenericAlias`` objects (GH-29335)

The documentation on ``GenericAlias`` objects implies at multiple points that
only container classes can define ``__class_getitem__``. This is misleading.
This PR proposes a rewrite of the documentation to clarify that non-container
classes can define ``__class_getitem__``, and to clarify what it means when a
non-container class is parameterized.

See also: initial discussion of issues with this piece of documentation in
GH-29308, and previous BPO issue [42280](https://bugs.python.org/issue42280).

Also improved references in glossary and typing docs. Fixed some links.

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@innova.no>
Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>