Implement Doxygen for CMake

The following discussion from !148 (merged) should be addressed:

• @lmoureau started a discussion:

  Implement Doxygen

changed milestone to %Alma9 migration

assigned to @lmoureau

See https://gitlab.cern.ch/cms-analysis/general/DasAnalysisSystem/campaigns/gitlab-profile/-/issues/1 too (fits quite well with the present issue).

added Ongoing label

added to epic &3

closed

mentioned in commit ccf72395