What would that look like? I assume you need something more sophisticated than git diff?

Since documentation is mostly text, so output to the like of "diff -y --left-column" is fine to me, with the different lines either inline or on the side.

You can use magit on a text selection/region

Magit is amazing but if you just want a git diff in emacs, the built-in vc-version-diff may also get the job done (vc is quite decent, check it out if you haven't!). (The question doesn't seem to be related to cider, so perhaps #emacs would have been a better place for it?)