Documentation for dendro-core

We need to include documentation for dendro-core. One question I have is whether we should keep this separate from the dendrodocs, which covers more than just the Python version.

I think we had decided before that actually it would be best to only really have to maintain one reference implementation, and that we would go with Python, so one thing we could do is have this repository have its own proper documentation as if it was an independent package, then we can move the dendrodocs to a separate repository in this organization, prominently feature the Python package, and include information about the IDL and C++ codes as alternatives, indicating that the main reference implementation is the Python one.

@ChrisBeaumont @bradenmacdonald @low-sky - what do you think? If that sounds reasonable to you I can start working on the documentation for the core package itself?