Change recommonmark for myst
Created by: scarlehoff
While it might not be the problem of #1537, this is something that we should've done some time ago given https://github.com/NNPDF/nnpdf/issues/1220#issuecomment-947732053
I think that the documentation produced by this branch is equal to the documentation in master (actually, a bit better because I've fixed a few broken links I've found) but I've only checked by eye.