For example, if I want to collaborate on making a (non-computer related) textbook with someone, and we want to use git as a version control system. Should we use github/codeberg for this or is there a platform that’s more suitable for using git in nontechnical contexts?
Plenty of people use git hosting to host non-code, like documentation, books written in markdown/LaTeX, etc. I personally use git to maintain a few personal wikis.
Different git forges will have different rules about what content they allow. GitHub definitely allows non-code. I’ve seen Codeberg repos be used for non-code too. Codeberg’s only requirement is that you only host free (as in freedom) content, so I suppose for non-code that means using an appropriate CC licence for example. I can’t imagine any of the popular git hosting sites taking issue with someone hosting their book, unless you’re hosting, like, the whole of Wikipedia or something.