Documentation on data specification should clearly describe the actual current specification

Created by: Zaharid

Currently we have

https://docs.nnpdf.science/vp/dataspecification.html?highlight=t0#

which makes historical sense as it was written to specify the transition to the "data keyword". But we lack a resource that focuses from the start on how to specify datasets, details on covmats filters (i.e. links to the existing page) and so on, a bit along the lines of what we have in the paper.

At minimum we should move things around so the first section concerns the current data.