Consider switching to mkdocs

[fdiblen]

We should consider switching to MkDocs for a more modern setup.

I conducted a small test, and the transition is quite straightforward. You can check it out here: https://github.com/fdiblen/mkdocs-pdf

[bouweandela]

Will this help with #338? I saw that readthedocs supports mkdocs and I know that it supports and previews for pull requests, so maybe we could use that if we cannot get it to work with github pages?

[egpbos]

Would it also make sense to use https://github.com/esciencecenter-digital-skills/NEBULA?