Generate API docs for Sphinx

Generate API docs from the Python package in Sphinx to add to the [ReadTheDocs documentation](https://itkpythonpackage.readthedocs.io/en/master/).

Start by locating the Python package and the Sphinx/ReadTheDocs configuration in the repository. Determine how API documentation should be generated and included at the linked ReadTheDocs site. Done means the package API documentation is generated in Sphinx and added to the published documentation.

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