Some links to descriptions of using Git and Github for text-based writing projects.
Collaborative Writing on GitHub By Ole Begemann February 29, 2016
I was involved as an editor in several collaborative writing projects in the past year: I edited two objc.io issues, and I was the technical reviewer for the Core Data and Advanced Swift books. I’d like to tell you a bit about my experiences. ... The editing process for the books worked like this: The authors would write chapters in Markdown and push directly to a private repository on GitHub.
Synchronized editing: the future of collaborative writing
By Jeffrey M. Perkel - 31 March 2020
A growing suite of tools allows teams of researchers to work collectively to edit scientific documents.
Botvinnik managed her paper as if it were open-source software. She wrote it in a plain-text editor and placed text files alongside data sets and code for generating figures on the code-sharing site GitHub. She invited her four co-authors to submit edits using Git, software that tracks precisely how and when a file has been changed. And she used a dedicated tool called Manubot to render the document as a user-friendly manuscript, which she then published online and tweeted to the world.