Get rid of `recommonmark` dependency

Created by: RoyStegeman

As discussed in https://github.com/NNPDF/nnpdf/issues/1970 recommonmark is no longer being maintained, and we should no longer depend on it. @achiefa has agreed to solve this by updating all .md files in our docs to .rst format.

See https://github.com/NNPDF/nnpdf/pull/1597 for an earlier attempt