I'm still not sure. Never set up a wiki on my own to be honest.
Is there a hoster or place you would prefer or advise?
the best place would be on the official website, if they have such features where users can write a tutorial. it takes a lot of time and often developers don't have time to write it themselves.
Developers should document new features, but it's rarely done (I never did it, and ended explaining the same thing multiple time on the forum instead of writing it once ahaha)
letting users who know the program very well write the guide is one less work to do from the developers.
If they don't have a wiki of their own, we can always use wikitemp. I know it's not used often by users, except for compatibility list, but promoting it with guides could help improve its coverage. (not sure I used the correct word here).
the forum has a lot of old outdated tutorial, they can't be updated because the author is not logging anymore. if wikitemp was more known as a place to find guides, maybe it would help maintaining these guides.
That's just an idea. Maybe you don't like the idea of writing it on wikitemp, but you already wrote your guide on gbatemp, so it's the same place.
if you need help with wiki, you can ask. I can create the needed pages or even some templates if you don't know how to do it.
If you want to host your own wiki engine, it will cover only retroarch ? it could be cleaner to make it only for retroarch instead of part of another website like wikitemp, but I don't have any suggestion for a web host. I know only OVH (it's french).