add myst_parser to allow markdown files in toclist in docs

[egpbos]

Adds the sphinx plugin myst_parser. This allows you to add Markdown files directly to the toclist in index.rst. That way an unnecessary manual transformation step from Markdown to RST is avoided.

[egpbos]

@sverhoeven Good suggestion. Personally, I put most markdown files under docs in my projects, because I don't like the root directory to be too stuffed. So, to showcase markdown support, I could do that here as well. Shall we discuss this further in a new issue and merge this?