docs(langgraph): add integration guide for LangGraph (#1422)

* docs(langgraph): add integration guide for LangGraph

* doc edit (#1423)

---------

Co-authored-by: Miyoung Choi <[email protected]>

Author: Pouyanpi

docs/user-guides/langchain/index.rst

@@ -6,5 +6,6 @@ LangChain
 
    langchain-integration
    runnable-rails
+   langgraph-integration
    chain-with-guardrails/index
    runnable-as-action/index