Add API Documentation

Create comprehensive API documentation covering:
- Core functions and classes
- Parameters and return types
- Usage examples
- Error handling

Example additions:
```markdown
## API Reference
### get_sage(provider, model, base_url=None, api_key=None, mode='default')
Creates a new ShellSage instance with specified configuration.

Parameters:
- provider: str - The AI provider to use ('anthropic' or 'openai')
- model: str - Model name to use
```

Estimated time: 3-4 hours