bpo-42128: Add documentation for pattern matching (PEP 634) (#24664)

This is a first edition, ready to go out with the implementation. We'll iterate during the rest of the period leading up to 3.10.0.

Co-authored-by: Carol Willing <carolcode@willingconsulting.com>
Co-authored-by: Fidget-Spinner <28750310+Fidget-Spinner@users.noreply.github.com>
Co-authored-by: Brandt Bucher <brandt@python.org>
Co-authored-by: Raymond Hettinger <1623689+rhettinger@users.noreply.github.com>
Co-authored-by: Guido van Rossum <guido@python.org>