✨ Add `sphinx.ext.apidoc` extension

[chrisjsewell]

A common use case, is that users want to simply point sphinx towards a Python module, and have it generate documentation automatically. (see e.g. https://github.com/rtfd/readthedocs.org/issues/1139)

This is not possible currently, without a "pre-build" step of running the sphinx-apidoc CLI. (and in fact there are numerous examples of users calling sphinx-apidoc within their conf.py to work around this)

This PR adds sphinx.ext.apidoc as a sphinx extension, to incorporate the source file generation into the sphinx build in a "formal" manner, using the API rather than the CLI.

related to #6829 (but does not close it)

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This PR is currently in draft, since there are a number of TODOs in the code, and documentation needs to be added.