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

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

docs/contributing/DOCS_GUIDE.md

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+# Documentation Build Guide
+
+This directory contains the source files for the vLLM-omni documentation.
+
+## Building Documentation Locally
+
+### Prerequisites
+
+Install documentation dependencies:
+
+```bash
+pip install -e ".[docs]"
+```
+
+### Build and Serve Documentation
+
+From the project root:
+
+```bash
+# Serve documentation locally (auto-reload on changes)
+# This starts a local web server at http://127.0.0.1:8000
+mkdocs serve
+
+# Build static site (generates HTML files in site/ directory)
+mkdocs build
+```
+
+When using `mkdocs serve`, the documentation will be automatically available at `http://127.0.0.1:8000`. The server will automatically reload when you make changes to the documentation files.
+
+## Auto-generating API Documentation
+
+The documentation automatically extracts docstrings from the code using mkdocstrings. To ensure your code is documented:
+
+1. Add docstrings to all public classes, functions, and methods
+2. Use Google or NumPy style docstrings (both are supported)
+3. Rebuild the documentation to see changes
+
+Example docstring:
+
+```python
+class OmniLLM:
+    """Main entry point for vLLM-omni inference.
+
+    This class provides a high-level interface for running multi-modal
+    inference with non-autoregressive models.
+
+    Args:
+        model: Model name or path
+        stage_configs: Optional stage configurations
+        **kwargs: Additional arguments passed to the engine
+
+    Example:
+        >>> llm = OmniLLM(model="Qwen/Qwen2.5-Omni")
+        >>> outputs = llm.generate(prompts="Hello")
+    """
+```
+
+## Documentation Structure
+
+```
+docs/
+├── index.md              # Main documentation page
+├── getting_started/      # Getting started guides
+├── architecture/        # Architecture documentation
+├── api/                 # API reference (auto-generated from code)
+├── examples/            # Code examples
+└── stylesheets/         # Custom CSS
+```
+
+## Publishing Documentation
+
+### GitHub Pages (Recommended)
+
+The documentation is automatically deployed to GitHub Pages using GitHub Actions.
+
+1. **Enable GitHub Pages**:
+   - Go to repository `Settings` → `Pages`
+   - Set `Source` to `GitHub Actions`
+   - Save settings
+
+2. **Push changes**:
+   ```bash
+   git push origin main
+   ```
+
+3. **Documentation will be available at**:
+   - `https://vllm-project.github.io/vllm-omni/`
+
+The GitHub Actions workflow (`.github/workflows/docs.yml`) will automatically:
+- Build the documentation when you push to `main` branch
+- Deploy it to GitHub Pages
+- Update the documentation whenever you make changes
+
+
+### Read the Docs (Alternative)
+
+You can also use Read the Docs for hosting:
+
+1. Sign up at https://readthedocs.org/
+2. Import the `vllm-project/vllm-omni` repository
+3. Read the Docs will automatically build using `.readthedocs.yml`
+4. Documentation will be available at: `https://vllm-omni.readthedocs.io/`
+
+## Configuration
+
+The documentation configuration is in `mkdocs.yml` at the project root.
+
+## Tips
+
+- **API Documentation**: API docs are automatically generated using `mkdocs-api-autonav` and `mkdocstrings`
+  - No need to manually create API pages - they're generated automatically
+  - Use `[module.name.ClassName][]` syntax for cross-references in Summary pages
+- **Code Snippets**: Use `--8<-- "path/to/file.py"` for including code snippets
+- **Markdown**: Use Markdown for all documentation (no need for RST)
+- **Material Theme**: Use Material theme features like:
+  - Admonitions: `!!! note`, `!!! warning`, etc.
+  - Code blocks with syntax highlighting
+  - Tabs for organizing content
+  - Math formulas using `pymdownx.arithmatex`
+
+## Troubleshooting
+
+### Documentation not updating
+
+- Make sure you've saved all files
+- If using `mkdocs serve`, it should auto-reload
+- Check for syntax errors in `mkdocs.yml`
+
+### API links not working
+
+- Ensure class names match exactly (case-sensitive)
+- Check that the module is imported correctly
+- Run `mkdocs build --strict` to check for errors
+
+### Build errors
+
+- Check Python version (requires 3.9+)
+- Ensure all dependencies are installed: `pip install -e ".[docs]"`
+- Check `mkdocs.yml` syntax with `mkdocs build --strict`