Improve "Extending Python with C or C++" docs

Quoting Petr in issue gh-93078:

Anyway, the more important thing to do here is converting the tutorial. That will, in my opinion, make the new way official :)
But I don't want to do that until PEP 687 is accepted, and more PEP 630 open issues are solved. (FWIW: Metaclasses and Per-class scope look like they could go in anearly 3.12 alpha, Type checking should hopefully go in 3.12 as well, and lossless conversion isn't blocking for me.)

But yes, all that is out of scope of this PR.

Originally posted by @encukou in https://github.com/python/cpython/issues/93078#issuecomment-1135579802