Convert MD docs to RST

[michaelschwier]

MD does not render very nice with Sphinx, so we should convert all MD files to RST format at some point.

[michaelschwier]

Actually, RST is also not much better (e.g. for the contribution page), so not worth the effort to convert for now.

We should think if we should strive for one format just for consistency in the docs (then must be RST, because the autodoc generation from source code documentation requires RST).

This issue is NOT URGENT AT ALL