create sphinx documentation for widgyts

[munkm]

This PR should be v1 of our documentation! Woooo. These docs contain

• contributing guidelines
• installation instructions for pypi, from source, and development builds
• directions on building the docs locally
• a new example notebook that shows how to use the FRBViewer widget with yt but without the monkeypatching. It also shows the traitlet linking
• basic API documentation
• a code of conduct (with attribution on the lineage of this documentation)

Other notes:

• the testing page is a placeholder. This will be filled out once the test suite is created/built

This PR resolves data-exp-lab/widgyts#33 and data-exp-lab/widgyts#37. However, some of the checkboxes on 37 are minimally-solved. The docstrings need to be updated so that the API docs for autodoc are a bit nicer. Also it would be nice to eventually update our docs so the notebooks render in the documentation with jupyter sphinx.

[matthewturk]

once the two minor nits are done let's merge