gh-134160: Split extension module init from PyModule docs; emphasize multi-phase init

[encukou]

#134764 moved a section around; I don't think that's enough.

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• Split extension module init from PyModule docs; emphasize multi-phase init

  There's a difference between working with module objects in general
  (which just needs garden-variety API docs), and defining the init
  function (which covers things like correctly exporting a function
  defined by the user, and lots of behaviour that's unrelated to generic
  module objects).

• Document behaviour of single-phase init. Call it "legacy".

• Reorganize PyModule docs.

• Move PyInit_modulename docs from the tutorial, and PyMODINIT_FUNC docs
  from the generic macros section, to the new page.

• Add docs

• Add doc stubs for PYTHON_API_VERSION & PYTHON_ABI_VERSION

• Remove incorrect refcounts.dat entry for PyModuleDef_Init;
  instead note that the function sometimes returns a borrowed reference,
  sometimes as strong one.
  (IMO, it's best to pretend PyModuleDef isn't a PyObject at all,
  but, reference docs should be correct.)

• Issue: Docs Examples Focus on Outdated Single-Phase Init Modules and Static Types #134160

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📚 Documentation preview 📚: https://cpython-previews--135126.org.readthedocs.build/

[StanFromIreland]

From a quick look, though I assume some are existing errors.

[encukou]

Thank you!