Documentation for flux headers

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:

• Darcys law (!3177 (merged))
• Ficks law (@leonidas, !3267 (merged) )
• Forchheimers law (@leonidas, !3231 (merged) )
• Fouriers law (@leonidas, !3269 (merged) )
• Hooks law (@leonidas, !3270 (merged) )
• Maxwell Stefans law (@stefaniekiemle, !3297 (merged))
• dispersion (@stefaniekiemle, !3299 (merged))
• effective stress law (@leonidas, !3229 (merged) )
• Shallowwater (@leopold.stadler , !3301 (merged) )
• Shallowwaterviscous (@utz , !3301 (merged))
• Fourierslaw non equilibrium (@stefaniekiemle, !3314 (merged) )