well there are wikis, but that may be a bit overkill, and there aren't many free wiki hosters left nowadays.

perhaps more suitable would be collaborative editing/note-taking platforms. there are the open source ones like etherpad and hedgedoc, where you can choose (or even self-host) your own instance (here's the public instance list for etherpad and hedgedoc). there's also notion, which I think the admin team here uses, so you can probably ask them for more information, but that's a centralized platform.

edit: by the way, what resources are you using to learn git? I enjoyed reading this, and found it helpful (it helps that at least one of the examples they use is a story writing one, and the software itself is more interactive out-of-the-box than git) even though it is not exactly git (but still in the same category; windows and macosx binaries available at official site in case you find it interesting).

edit 2: anyway, I feel distributed version control is still best learnt collaboratively (it was invented to solve such a problem anyway), so if you ever get bored of typing commands into the terminal, let me know.