bpo-45655: Add "relevant PEPs" section to ``typing`` documentation (GH-29280)

The list of PEPs at the top of the documentation for the ``typing`` module has
become too long to be readable. This PR proposes presenting this
information in a more structured and readable way by adding a new "relevant
PEPs" section to the ``typing`` docs.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>