More documentation

[ivirshup]

The documentation for this package is currently largely limited to reference docs. There is also a tutorial notebook, but it is mainly a listing of the available methods.

I would recommend adding more explanation and how-to guides, following the diataxis framework.

[brianraymor]

Thanks for sharing the reference. I plan to review. I'd note that the original primary intent for this package was for internal scenarios to enforce consistency - to eliminate duplication of ontology downloads and processing across CELLxGENE experiences. In the past, we've had cases where CELLxGENE applications/services were not using the pinned ontology releases in the schema.