docs: enhance migration path docs for variable listeners

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.

Locate the documentation sections covering shadow variables and the older VariableListeners approach, then compare how the existing migration guidance is organized. Add migration guidance for cases that use Working Solution problem facts, including the suggested placement on Values or Entities, and confirm the new section clearly explains when each approach applies.