docs(zenoh): Add usage examples and API reference

[zhexuany]

Documentation Task

Add usage examples and API reference documentation for the Zenoh plugin.

Proposed Documentation

1. Quick start guide with code example
2. API reference for all public methods
3. Example code for common patterns
4. ROS2 integration example

Files to Add

• middlewares/zenoh/README.md
• middlewares/zenoh/examples/
• middlewares/zenoh/docs/api_reference.md

Priority

Medium - Documentation helps adoption