Documentation: Improve planning problem modeling guide

Coming up with a good data model is a fundamental step of solving a planning problem using OptaPlanner. There are some subtleties that need to be taken into account, e.g. Following best practices of object-oriented design, correctly identifying planning entities & variables, using appropriate data types, etc.

The documentation already contains description of how to identify planning entities https://docs.optaplanner.org/latestFinal/optaplanner-docs/html_single/#isThisClassAProblemFactOrPlanningEntity

We should promote & enhance the section as this is most likely one of the first issues a new user experiences.
Apart from general principles, it might be worth to see their application to individual examples OptaPlanner ships with to answer questions like "why it's X what's planning entity in example Y and not Z" - if there are any steps where the users usually go wrong, let's include them. Including ideas about extensibility of the examples seems to be important as well - "I've got example X, where do I go now? What are the options to extend the example to fit my needs?" Some pointers are always helpful.