docs: enhance migration path docs for variable listeners

[TomCools]

With the new approach to create shadow variables being promoted in the docs, we could add a section on the old VariableListeners on how to migrate some cases.

e.g.: Some people use the Working Solution in their variable listeners, to find some Problem Facts to calculate the shadow variables with. A fair mitigation would be to add those problem facts either on the Values or on the Entities themselves, depending on context.