Add markdown support for the documentation

[matrxs]

Markdown is an easier and more intuitive method of writing Richt Text, compared to the RichStructuredText (.rst) format. Markdown is also often used for wiki's and readme's of code repositories.

As such, it would ease the creation of new documentation pages a lot if markdown was supported.

[thaije]

https://docs.readthedocs.io/en/stable/intro/getting-started-with-sphinx.html#using-markdown-with-sphinx