Generate API documentation

[tbirdso]

The itkwidgets documentation at https://itkwidgets.readthedocs.io/en/latest/deployments.html is useful for getting started, but does not seem to provide a comprehensive overview of the itkwidgets Python API. Existing documentation provides "common flags" for CLI development, which implies that more flags are available for advanced use. Meanwhile, it's not clear what keyword inputs are available for itkwidgets.view, etc.

It would be helpful to have comprehensive API documentation available as part of that documentation. It may be reasonable to generate that API documentation if docstrings are available inline.