GitHub's wikis are just git repositories with folders full of Markdown files. That's the easiest thing to pull out of GitHub. Just git clone the wiki repo. At that point I often just use a git headless merge to merge the second repo into the main repo under a docs/wiki/ folder or similar.

As I recall, in GHE I remember finding that you could even turn off the repo's wiki at that point and then reset it to point to your docs/wiki/ folder and still have the Wiki interface but pointing it to a branch in the same repository. That was a while ago and I don't know if it is still supported in GHE. I also remember it was work managing that wiki branch because Wikis didn't directly support PR workflows if you have branch protection policies. Also from the documentation it doesn't look like it is supported in Public GitHub right now, which seems a shame to me, as someone who likes all documentation in the same repo as the source. I haven't needed Wikis in public GitHub much, so I haven't tried to do anything like point them to a main repo branch.

That's fair. You could also just put the wiki in a different branch if that's your style.