[3.10] bo-45655: Add "relevant PEPs" section to typing documentation (GH-29302)

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.

(cherry picked from commit 03db1bbfd2d3f5a343c293b2f0e09a1e962df7ea)

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