Add Doxygen generation using CMake

This is a first configuration and probably requires adjustments to be portable.

Doxygen warnings are saved in the ${PUGS_BINARY_DIR}/doxygen.warnings This file will eventually be used to check that documentation is complete.

Closes #12 (closed)

enabled an automatic merge when the pipeline for 13914155 succeeds

mentioned in commit 0a259f7e

merged