Postfit docs
Created by: voisey
As talked about in https://github.com/NNPDF/nnpdf/pull/1023 I've started to add more comprehensive docs on postfit. Note that these should eventually be linked from the pages on how to run both nnfit and n3fit fits.
@tgiani Could you possibly add some information on the integrability and positivity vetoes please? I've left some blanks where certain information should go.