bpo-40939: Remove documentation for `PyParser_*` & add porting notes (GH-26855) (GH-26898)

I tried to be relatively thorough and give lots of links.
One reason is that this wasn't deprecated very long; also it seems people running into this tend to not be familiar with similar APIs.

(cherry picked from commit 29987f72650b7cccee4df216c8297e8484a44e6a)

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>