docs: rewrite LangChain integration guide (#24)

Complete rewrite of the langchain-capiscio integration page:

- Zero-config CapiscioGuard() quickstart
- LCEL pipe composition (guard | agent)
- Enforcement modes (block/monitor/log)
- Explicit configuration and connect_kwargs
- CapiscioCallbackHandler usage
- LangGraph decorator and node patterns
- Badge token extraction from context/config/input
- Tool-level enforcement with CapiscioTool
- Full working example
- API reference for all public exports

Author: beonde