Part of #1139

• that's a basic overview of internals with uml diagrams (based on plantuml)
• PMD high-level building blocks (impl, such as: threading, phases, concepts)
• Depends partly on [[PMD 7.0.0 API|PMD-7.0.0-API]] because of the naming of the concepts
  and API

Maybe use https://arc42.org/overview as a guideline to document the architecture.

With #4072 we have part 9: "Architectural decisions"