Transform documentation

[mmacata]

It would be more feasable to use markdown instead of rst. Therefore it would be good to transform the existing documentation and integrate it into CI, possibly use https://www.mkdocs.org, https://github.com/mkdocs/mkdocs/wiki/MkDocs-Themes.
While at it it would be also great to restructure existing documentation (API docs, user docs, docstring, tutorial, ...) to avoid rendundancy.