docs: add comprehensive Java API documentation

- Add README-java.md with full Java API reference
- Add link to Java docs from main README.md
- Simplify examples/java/README.md to compilation instructions
- Update examples/README.md

README-java.md includes:
- Installation (Maven)
- Quick start examples
- Full API reference for all interfaces and classes
- Configuration options
- Async patterns (CompletableFuture, EventSubscription)
- Custom tools and permission handling
- Event types reference

Author: krukow