Add a "go from applgrid/fastnlo table to an NNPDF fktable" tutorial

[scarlehoff]

The necessary steps are:

1. Use PineAPPL to convert the grid into an PineAPPL grid (and have it be part of the pinefarm Add conversion backend pinefarm#3)
2. Select your NNPDF theory https://github.com/NNPDF/nnpdf/tree/master/nnpdf_data/nnpdf_data/theory_cards#database-of-theories-implemented-in-the-nnpdf-framework
3. Run pineko (this would be needed Allow to run explicitly a grid / fktable #168, otherwise a yamldb / nnpdf dataset need to be used)

[alecandido]

Since Pineko is only a piece, and not the full Pineline, I'd propose to publish the tutorial on the overarching website

https://nnpdf.github.io/pineline

and transfer this issue to corresponding repo https://github.com/NNPDF/pineline.

(notice that neither pineappl import neither the theories are intrinsically part of Pineko, but they are part of the Pineline)