Switch MarkDown parser

[cboulanger]

The currently used markdown parser seems to be buggy and unmaintained (see #676); it should therefore be replaced with a different implementation. Possible alternatives:

• https://github.com/markdown-it/markdown-it
• https://marked.js.org/
• https://github.com/remarkjs/remark

[hkollmann]

We should use remark - same as in @qooxdoo/documentation.