fix: quickstart code formatting and indentation

Add prettier-ignore to prevent linter from stripping indentation
inside code blocks. Break long lines for narrow docs column.
Filter empty messages in onMessageReceived callback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: khaliqgant

web/content/docs/quickstart.mdx

@@ -1,6 +1,8 @@
----
+## {/* prettier-ignore */}
+
 title: 'Quickstart'
 description: 'Launch your first agents in 2 minutes.'
+
 ---
 
 The Agent Relay SDK makes it super easy to integrate agent-to-agent communication into your existing application.