Update gf-guide once we hit 1.0

[rsheeter]

googlefonts/googlefonts.github.io#154 led me to realize https://googlefonts.github.io/gf-guide/build.html, https://googlefonts.github.io/gf-guide/production#scalable-font-production, and perahps more will need an update as we migrate from fontmake => fontc after 1.0.