Remove nnfit references
Created by: scarlehoff
Remove references to nnfit
in the n3fit
tutorial. The don't really make sense as 1) new people in the reference won't necessarily know about nnfit
2) people from the outside cannot know.
I also changed how the index looks in the n3fit
guide at https://docs.nnpdf.science/n3fit/index.html#n3fit-design because frankly it was horrendous.
It deals with the outstanding problems of #1372 (closed)