Add "physics" documentation (discretization-agnostic) for the fluxes (e.g. Darcy's law / Fick's law / ...). This can then be referenced by the model documentation. Todo: - [x] Darcys law (!3177) - [x] Ficks law (@leonidas, !3267 ) - [x] Forchheimers law (@leonidas, !3231 ) - [x] Fouriers law (@leonidas, !3269 ) - [x] Hooks law (@leonidas, !3270 ) - [x] Maxwell Stefans law (@stefaniekiemle, !3297) - [x] dispersion (@stefaniekiemle, !3299) - [x] effective stress law (@leonidas, !3229 ) - [x] Shallowwater (@leopold.stadler , !3301 ) - [x] Shallowwaterviscous (@utz , !3301) - [x] Fourierslaw non equilibrium (@stefaniekiemle, !3314 )