Set up documentation

Easiest is use use ReadTheDocs, but this will still require setting up landing/description pages etc.

Even though we're not super strict about our versioning yet (we always just bump the minor version and break backwards compatibility because we are <v1.0), it's nevertheless useful to know about the state of the software at a given version.

In addition we can ship versions of the example notebook that correspond to our released version. This would prevent confusion e.g. #90 (closed).