Documents in this directory use variables defined in cmake. You have to run doxygen command after cmake replaces these variables. cmake defines a target for running doxygen:

cd /path/to/monolish/repo/top/
cmake -B build .                        # Generates build/doc/* files
cmake --build build/ --target document  # Run doxygen at build/

If you want to check generated markdown files, you can find them at build/doc/.