docs: add o11y overview (#5542)

* docs: add o11y overview

* add section for enabling tracing

docs/docs/how-tos/observability.md

@@ -0,0 +1,21 @@
+# Observability
+
+Use [LangSmith](https://smith.langchain.com/) to visualize the execution of a LangGraph application and do the following:
+
+- [Enable tracing for your application](#enable-tracing-for-your-application).
+- [Debug a locally running application](../cloud/how-tos/clone_traces_studio.md).
+- [Evaluate the application performance](../agents/evals.md).
+- [Monitor the application](https://docs.smith.langchain.com/observability/how_to_guides/dashboards).
+
+To get started, sign up for a free account at [LangSmith](https://smith.langchain.com/).
+
+## Enable tracing for your application
+
+To use LangSmith with your LangGraph application, enable tracing:
+
+```python
+export LANGSMITH_TRACING=true
+export LANGSMITH_API_KEY=<your-api-key>
+```
+
+For more information, see [Trace with LangGraph](https://docs.smith.langchain.com/observability/how_to_guides/trace_with_langgraph).