gh-140550: PEP 793 reference documentation (GH-141197)

* gh-140550: PEP 793 reference documentation

Since the PEP calls for soft-deprecation of the existing initialization
function, this reorganizes the relevant docs to put the new way of
doing things first, and de-emphasize the old.

Some bits, like the tutorial, are left out of this patch. (See the
issue for a list.)