docs: List LangSmith tracing integration

Author: baskaryan

Diff for: docs/tracing.md

@@ -95,3 +95,4 @@ External trace processors include:
 -   [AgentOps](https://docs.agentops.ai/v1/integrations/agentssdk)
 -   [Scorecard](https://docs.scorecard.io/docs/documentation/features/tracing#openai-agents-sdk-integration))
 -   [Keywords AI](https://docs.keywordsai.co/integration/development-frameworks/openai-agent)
+-   [LangSmith](https://docs.smith.langchain.com/observability/how_to_guides/trace_with_openai_agents_sdk)