Merge branch 'cleanup/doxygen-for-3.0-porousmediumflow' into 'master'

Cleanup/doxygen for 3.0 porousmediumflow

Closes #596

See merge request !1416

(cherry picked from commit 72589d16)

55a2be36 [doxygen] Adapt documentation for 1p, 1pnc, 1pncmin models
47f93884 [doxygen] Adapt documentation for 2p, 2p1c, 2p2c, 2pnc, 2pncmin models
df53d24c [doxygen] Adapt documentation for 3p, 3p3c, 3pwateroil models
e69730ba [doxygen] Adapt documentation for boxdfm, co2, mpnc, mineralization models
7784c756 [doxygen] Adapt documentation for richards, richardsnc, tracer models
a06acdf4 [doxygen] Adapt documentation for nonequilibrium, nonisothermal models
57faf61d [doxygen] Adapt documentation for compositional, immiscible models and general
bd292ebf [doxygen] Adapt modules.txt and handbook.bib
76f567b6 [doxygen] Adapt documentation in 1p sequential
b9b3d5c6 [doxygen] Adapt documentation in 2p sequential
921a3b36 [doxygen] Adapt documentation in 2p2c sequential