Publish API documentation

[ewoutkramer]

We have done quite a lot of work on documenting the public interface of the SDK by now. It's by no means complete, but all recent functionality is fully documented, and we work on older bits when we revisit/refactor them.

I think it is time we start publishing the API documentation on our readthedocs website, and to start doing that we need to turn the doc comments in our source code into html.

Gino suggests using docfx. He managed to set it up as a GH workflow with pages (docs.yaml). The generated site is here.

Is this a candidate for us to use?