Create a documentation page according to diataxis principle

[mknorps]

The current README.md contains more and more information in most if the areas of diataxis principles: how-to guides, explanation and reference. We do not yet have tutorials, and may consider them when FawltyDeps is more mature.

The README becomes too big to extract the information fast. I think at this point we need a separate way and place to serve documentation.

We could use mkdocs and deploy as GitHub pages.