[doxygen] Cleanup/doxygen for 3.0 tests

See merge request !1404

(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