Migrate to mkdocs

[jh-RLI]

Mkdocs now is a popular technology in our framework. We considered to switch the documentation platform form ReadTheDocs to a github hosted mkdocs integration. As it is more usable to create documentation from simple markdown syntax.

Nethertheless most of the sphinx functionality should be added by utilizny integration packages if they are available (on pypi).

[jh-RLI]

I will start by testing how easy it is to convert the RST files of the current documentation into pure Markdown.