The approach I have taken is to host the documentation in Github. This has several advantages over other approaches.
On each page of the mendix application, there is a link to a corresponding document on Github for that particular functionality.
This link is saved in the database and I set up a special user role that can edit them on the spot. So far it has worked out for me.
The only minor inconvenience is that uploading images and videos can not be done directly with drag-n-drop. Instead, you have to first upload them then link them. Also, from a styling perspective it looks ok but not amazing.
Lovely. I talked about this with a colleague today. We use confluence, a well-known documentation tool. But text there gets written and forgotten, hardly ever read. The same goes for most documentation tools is my experience.
Two nice Appstore apps are “Help Text Viewer“ and the “IntroWidget”. The Introwidget is for creating an introduction video. The help text viewer is probably the one most matching your requirement. It is old though, V5 and need to get upgraded first.