Rework documentation by following Diátaxis

[Zimmi48]

It feels like we are growing out of a README-based documentation. I think we could apply the principles advocated in the Diátaxis framework (https://diataxis.fr/): split documentation into tutorials, how-to guides, explanation and reference. Except that I would focus on how-to guides and reference to start with, explanations next, and tutorials maybe at some point later.