Add Java client API documentation

[codelipenghui]

The documentation provides comprehensive information about the Oxia Java client API, including:

1. Basic usage examples for both synchronous and asynchronous clients
2. Client configuration options
3. Namespace management
4. Authentication
5. Conditional operations
6. Partition keys
7. Sequential keys
8. Secondary indexes
9. Key comparison operations
10. Range operations
11. Notifications
12. Ephemeral records
13. Error handling