Moving sphinx to app requirements and changing the build system to use sphinx out of the app tree is the first step.
Then, we need to add a way to for the devel command to generate documentation as well so it is included in the bundle. The user documentation is expected to be placed in the bundle source in src/docs/user/commands/CMD.html and src/docs/user/tools/TOOL.html. And <DataDir>docs</DataDir> placed in the bundle_info.xml file. Those command and tools will be automatically added to the user documentation index. Developer API information could be placed in the src/docs/devel/SOMEWHERE, but there is no automatic integration with the rest of the developer documentation, so there is no advantage of putting it there yet.