Document custom types, holder pattern and binary compatibility test process

[gbarnett-hz]

For hazelcast/hazelcast#25223 there were a number of scenarios encountered that had no documentation:

• When using types specific to a target language you need to create holder types as an intermediate representation from the protocol types
• Placement of custom types and their mappings in various places
• Binary compatibility tests and how they relate to other projects, e.g. ReferenceObjects.java in Hazelcast OS project
• Implied assumptions of generated tests on the domain types