docs: add comprehensive bilingual README documentation (#9)

Transform README.md into comprehensive documentation with complete API reference, advanced examples, and bilingual support. Add README_CN.md for Chinese language documentation.

Key improvements:
- Complete API documentation with all core classes and methods
- Advanced usage examples including streaming and custom LLM providers
- Detailed variable system explanation with practical examples
- Ecosystem integration and consistent branding with markdown-flow-ui
- Bilingual support for broader accessibility

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: yfge