Generating Documentation With Doxygen - LMMS/lmms GitHub Wiki
Dependencies
Along with the usual dependencies for building LMMS you will also need Doxygen.
Generating the Documentation
Assuming you've already cloned the source code, setup the build/ directory, and ran cmake and are also in lmms/build/ (where the previously linked pages will leave you) directory run the following command:
cmake --build . --target doc
Documentation will be generated in lmms/build/doc/doc/.