This is the first time I've seen a github used for non programming purpose

- German federal law and regulations: https://github.com/bundestag/gesetze

- DMCA takedown notices: https://github.com/github/dmca

- Map of US districts: https://github.com/benbalter/congressional-districts

- a novel: https://github.com/JJ/hoborg

- a well-known textbook on some niche (homotopy type theory) in mathematics: https://github.com/HoTT/book

- fonts: https://github.com/theleagueof

- Gregorian chants: https://github.com/CMAA/nova-organi-harmonia

Main collaboration functions on a 'repository' (or any set of data / documents that somehow belong together) such as forking and branching are not just only relevant to programming.

(edit: formatting)