Expand documentation on building for the Limited API

[bgilbert]

PyPUG has a small section on extension module API compatibility, which links to Python's nice Limited API documentation. The Python docs imply that the Limited API can be used simply by setting Py_LIMITED_API and auditing API calls in the code, but there are some packaging concerns as well. As far as I can tell, enabling the Limited API requires setting three separate knobs:

1. Py_LIMITED_API, to hide non-stabilized symbols and potentially replace some macros with functions.
2. The setuptools Extension(..., py_limited_api=True) flag, to use the correct filename for the extension module.
3. The wheel py-limited-api option, to correctly ABI tag the wheel. See also: https://github.com/pypa/wheel/issues/583

These knobs are distributed across the ecosystem and there doesn't seem to be any unified documentation for them. This blog post was helpful for general understanding, but doesn't provide fully actionable advice. A concrete guide would really help.

In addition, perhaps the docs could mention how to conditionally disable the Limited API when building for older Python releases. Packages may want to continue shipping version-specific wheels for releases that haven't stabilized all of the C functions they need. I used setup.py code like this:

_abi3 = sys.version_info >= (3, 11)
setup(
    ext_modules=[
        Extension(
            'fizzbuzz', ['fizzbuzz.c'],
            define_macros=[('Py_LIMITED_API', '0x030b0000')] if _abi3 else [],
            py_limited_api=_abi3,
        ),
    ],
    options={'bdist_wheel': {'py_limited_api': 'cp311'} if _abi3 else {}},
)