Use mkdocs instead of default documentation

`mkdocs` supports more extensions and themes (See https://squidfunk.github.io/mkdocs-material/) and imho looks nicer. However, The default documentation allows looking at multiple versions using a drop down. With `mkdocs` we'll need to either add a separate markdown file with links to older versions or write some additional html / css / js to make that work. Any thoughts?

Start by reviewing the repository's current default documentation setup and the MkDocs Material reference linked in the issue. Decide how older-version navigation should work—links or custom HTML/CSS/JS; done means an agreed migration path that preserves access to older versions.

Written by the indexing model from the issue text.