Fully document Python API

[dstansby]

I'm currently using the Python API to create a viewer state that I can easily convert to a URL - very handy, but I'm finding it very hard to understand what I can do with the various classes and properties. It would be very helpful if there was comprehensive API reference docs explaining what classes are available, and what properties they have.

[jbms]

It isn't comprehensive but there is now https://neuroglancer-docs.web.app