Documentation for FLAME GPU 2 is hosted at docs.flamegpu.com this includes a user guide and API documentation, generated using Breathe, Exhale, Sphinx and Doxygen.
You can find the source for the user guide documentation on GitHub (FLAMEGPU/FLAMEGPU2-docs). You can view the Doxygen formatted FLAME GPU 2 API documentation by building it locally via CMake.
The FLAME GPU 1.x user guide is hosted at docs-v1.flamegpu.com.