[doc] Leverage the fact that the actual types can now be indexed for typing (GH-22340)

This shows users that they can use the actual types. Using deprecated types is confusing.

This also prefers colections.abc.Sized instead of the alias typing.Sized. I guess the aliases were created to make it convenient to import all collections related types from the same place.

This should be backported to 3.9.

Automerge-Triggered-By: @gvanrossum