Fix broken xref in Next steps section

Replace xref to ai-gateway.adoc (not available in labs build) with
a direct URL to the published docs page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: micheleRP

ai-agents/langchain-agent/README.adoc

@@ -265,5 +265,5 @@ Stop the agent by pressing kbd:[Ctrl+C] in the terminal.
 ifdef::env-site[]
 == Next steps
 
-* xref:ai-agents:ROOT:ai-gateway.adoc[]
+* https://docs.redpanda.com/redpanda-cloud/ai-agents/ai-gateway/[Learn more about the AI Gateway]
 endif::[]