Consolidate desired upstream repo structure, provide golden exemplar

[davelab6]

@graphicore reminded me that there have been a dozen attempts to define a desired upstream repo structure over the years, and this repo has 3-4 descriptions.

While it has been good to see different ideas bloom, we should consolidate all the good ideas and make a new "GF 2020 Upstream Model" with both a description and a golden exemplar.

I think we used Montserrat for this in the last couple of years, but perhaps a multi axis family would be better reference material.

[graphicore]

It looks like searching for AUTHORS.txt in this repo reveal all the relevant places: https://github.com/googlefonts/gf-docs/search?q=AUTHORS.txt&unscoped_q=AUTHORS.txt