Runcard description should discuss datasets

Created by: Zaharid

This page

https://docs.nnpdf.science/n3fit/runcard_detailed.html?highlight=runcard%20detailed

appears to not be discussing datasets selection at all. I think this is best implemented by linking to this page

https://docs.nnpdf.science/vp/dataspecification.html

(which needs to be fixed as per #1354 (closed) ) and expanding a bit on the options relevant for the fit.