gh-141004: Document Py_ARRAY_LENGTH macro

[Yashp002]

This PR adds documentation for the Py_ARRAY_LENGTH macro to the C API documentation, addressing issue #141004.

The documentation is placed in the "Useful macros" section of Doc/c-api/intro.rst and includes:

• A description of the macro's purpose (computing array length at compile time)
• A warning that it must be used with C arrays, not pointers
• An explanation that it's equivalent to sizeof(array) / sizeof((array)[0])

This macro has existed since Python 2.7 but was previously undocumented in the official C API documentation.

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📚 Documentation preview 📚: https://cpython-previews--141601.org.readthedocs.build/

• Issue: Document the entire C API #141004