gh-140550: PEP 793 reference documentation

[encukou]

I'll still need a few more passes on this, but here's this week's draft for PEP 793 reference docs.

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 PR. (See the issue for a list.)

• Issue: Implement PEP 793 – PyModExport #140550

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📚 Documentation preview 📚: https://cpython-previews--141197.org.readthedocs.build/

[ZeroIntensity]

LGTM

[encukou]

Thank you!