[doxygen] Cleanup/doxygen for 3.0 tests

See merge request !1404 (merged)

(cherry picked from commit 12b07f0b)

689919e1 [doxygen] Add/modify documentation aacfb75f [doxygen] Adapt documentation for all 1p related tests af703b06 [doxygen] Adapt documentation for all 2p related tests aa2a1769 [doxygen] Adapt documentation for all 3p related tests 16e2517f [doxygen] Adapt documentation for co2 tests 88c1b546 [doxygen] Adapt documentation for mpnc tests 303bac48 [doxygen] Adapt documentation for richards tests 9aeb02bb [doxygen] Adapt documentation for tracer tests 3ae3bf32 [doc][freeflow] Improve docu for freeflow tests 86fdd52a [doxygen] Adapt documentation for material tests f3b086d4 [doxygen] Fix more typos, blank spaces, etc. in tests folders 25269a4e [doxygen] Add missing citations in tests and handbook.bib e420306d [doxygen] Adapt documentation for multidomain tests 5e34c2c0 [doxygen] Adapt documentation for geomechanics tests 58da3381 [doxygen] Remove doxygen command in main files