[Docs] optimize and supplement docs system (#86)

Signed-off-by: Gaohan123 <gaohan19@huawei.com>

docs/design/api_design_doc.md

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+# API Design Documentation
+
+This document contains comprehensive API design documentation for all core modules in vLLM-omni. These templates provide a standardized structure for designing and implementing the core, engine, executor, and worker modules.
+
+## 📋 Module API
+
+### Core Module API
+**Core module** provides fundamental scheduling, caching, and resource management functionality.
+
+**Key Components:**
+- Request scheduling and prioritization
+- DiT cache management for diffusion models
+- Resource allocation and coordination
+- Inter-module communication
+
+### Engine Module API
+**Engine module** handles model loading, inference execution, and output processing.
+
+**Key Components:**
+- Model loading and initialization
+- Inference execution for AR and diffusion models
+- Input preprocessing for various modalities
+- Output postprocessing and formatting
+
+### Executor Module API
+**Executor module** coordinates and manages request execution across different engines and workers.
+
+**Key Components:**
+- Request routing and load balancing
+- Execution pipeline coordination
+- Worker management and task distribution
+- Error handling and recovery
+
+### Worker Module API
+**Worker module** provides the actual execution environment for model inference.
+
+**Key Components:**
+- Model execution and inference
+- GPU resource management
+- Request batching and processing
+- Performance optimization
+
+
+## 🔧 Implementation Guidelines
+
+### Code Standards
+- Use type hints for all methods
+- Include comprehensive docstrings
+- Follow PEP 8 style guidelines
+- Use async/await for I/O operations
+
+### Error Handling
+- Define specific exception types
+- Provide meaningful error messages
+- Include error recovery strategies
+- Log errors appropriately
+
+### Configuration
+- Use dataclasses for configuration
+- Provide sensible defaults
+- Include validation methods
+- Support environment variables
+
+### Testing
+- Write unit tests for all public methods
+- Include integration tests
+- Test error conditions
+- Validate configuration options
+
+## 🚀 Getting Started
+
+### For Developers
+
+1. **Choose a Module**
+   - Pick the module you want to work on
+   - Read the corresponding API documentation
+   - Understand the responsibilities and interfaces
+
+2. **Set Up Development Environment**
+   ```bash
+   # Clone the repository
+   git clone https://github.com/vllm-project/vllm-omni.git
+   cd vllm-omni
+
+   # Install dependencies
+   pip install -e ".[dev]"
+
+   # Set up pre-commit hooks
+   pre-commit install
+   ```
+
+3. **Start Implementation**
+   - Create the module directory
+   - Implement base classes first
+   - Add concrete implementations
+   - Write tests as you go
+
+### For Contributors
+
+1. **Review API Documentation**
+   - Read through all module APIs
+   - Understand the overall architecture
+   - Identify areas for improvement
+
+2. **Propose Changes**
+   - Create issues for API changes
+   - Discuss in pull requests
+   - Update documentation accordingly
+
+3. **Submit Contributions**
+   - Follow the coding standards
+   - Include tests for new features
+   - Update documentation
+   - Submit pull requests
+
+## 🤝 Contributing
+
+We welcome contributions to improve the API documentation and implementation. Please:
+
+1. **Follow the Template Structure**
+   - Use the [api template](https://github.com/vllm-project/vllm-omni/docs/contributing/design_documents/api_design_template.md) as a guide
+   - Maintain consistency across modules
+   - Include all required sections
+
+2. **Provide Working Examples**
+   - Test all code examples
+   - Include both basic and advanced usage
+   - Show integration patterns
+
+3. **Keep Documentation Updated**
+   - Update docs when APIs change
+   - Version control documentation changes
+   - Maintain backward compatibility notes
+
+## 📝 Notes
+
+- All API documentation should be kept up-to-date with implementation
+- Code examples should be tested and working
+- Configuration options should be validated
+- Error handling should be comprehensive
+- Performance considerations should be documented
+
+For questions or suggestions about the API documentation, please open an issue or start a discussion in the repository.