How to run a fit tutoriaal should link to runcard description

Created by: Zaharid

The tutorial on running fits

https://docs.nnpdf.science/tutorials/run-fit.html

which is going to be the first thing people read, is written to someone who knows about nnfit in detail. Instead it should be a starting point for how to run fits without assuming prior knowledge. The first sections should be written around a link to this very nice page

https://docs.nnpdf.science/n3fit/runcard_detailed.html

(with the additional nudge towards starting from an existing runcard).