How firm is the boundary between a dev doc and a user doc in your opinion? I have found that the overlap can be quite large if the users are also technically proficient. Right now I'm trying to balance "how X works so you can use the app better" with "how X works so you can contribute or build your own plugin". DeepWiki really helps as a backstop for anything not already covered though it's not without its own caveats of course.
Not OP but I think you have the right intuition in making a difference between using the app / contribute to the app. You may want to read https://diataxis.fr/ which elaborate on this idea and add another dimension (action / cognition) to this.