Documentation website

[FooBarWidget]

The documentation is currently in one huge README. I think it will look nicer, as well as be easier for users to navigate, if we turn that into a proper static site, with a sidebar as navigation. We could use Docusaurus for this purpose.

[PaulusTM]

I'm interested to help you out with this one.
Should we create this in a new repo?

[FooBarWidget]

Much appreciated. We can test things out in a new repo for now. In the end I'd like to see the work be put inside the main repo because this allows pull requests to cover code and documentation at the same time.