Feature/flux documentation of Darcy flux

What this MR does / why does DuMux need it:

This merge request improves the documentation (discretization-agnostic) for the fluxes (e.g. Darcy's law / Fick's law / ...). With that no description of the flux laws in every model is needed. We can reference to this description in every model the individual flux law is used.

Related to #1149 (closed)

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