* add module-level docstrings
* update compiler docstrings
* update registry module docstrings
* add BaseComponent and children to public api, but discourage use
* update artifact docstrings and type annotations
* update dsl docstrings
* update client docstrings
* clean up kfp.__init__
* add dsl placeholder docstrings
* many more docstring updates
* document type aliases in dsl module
* use __all__ in top-level modules to record public api
* add index and source files
* add kubeflow assets to _static/
* add and pin requirements
* use block quote instead of header for readme notice
* update conf.py
* delete old files