Initial setup for Doxygen generation

Summary: Create necessary CMake configuration to automate generation of Doxygen documentation

Test Plan:
Enabled generation of Doxygen with -DEVTGEN_BUILD_DOXYGEN=ON
Verified Doxygen pages were successfully generated

Reviewers: jback, kreps

Reviewed By: kreps

Tags: #evtgen

Maniphest Tasks: T98

Differential Revision: https://phab.hepforge.org/D51