As someone who recently worked on a Github wiki...it leaves much to be desired. The first being that I couldn't actually push with git! I needed collaborator access for that. I also find Github's markdown flavor to be limiting; you can do a lot more with a dedicated Wiki like MediaWiki. It was okay for viewing, though. And having the docs in an actual repository is much harder to navigate for users who aren't developers.

So while it's something you see a lot, I think they would be easier to collaborate on and view on a dedicated wiki.