Add guidance doc example of control with compound condition

[SouthEndMusic]

#1371 adds the ability to set control conditions on compound variables. As @visr pointed out, this adds more complexity to DiscreteControl which is already quite complex to schematize. Therefore I think it is good to have a realistic use case of this capability, maybe @gijsber can describe something below as he also wrote the original issue #1346.

Issue should answer the question: how to model a Ribasim7/8 reservoir node in Ribasim9