Front page of documentation

Created by: juanrojochacon

Currently the front page of the NNPDF code documentation

https://docs.nnpdf.science/

is not very informative, and it should be more self-contained with the description of what is NNPDF, what do we do, who are the members, links to the collaboration website etc. For example something along these lines would be better

https://lhcfitnikhef.github.io/SMEFT/

Also, the references are still missing, right now we only have @Zaharid paper ;)

I volunteer to propose a new layout for the front page of the documentation. I can try to open a pull request to see if people agree. But right now it is not very appealing, as front page of a huge and complex project as NNPDF is.

And the documentation is still missing, which makes very complicated to link the code with the papers where various stuff is developed. The docs will be specially important for the theory, K-factors etc, so the sooner we can have it ready the better