Add callout for credits/billing in AI Gateway Guide (#301)

* Add callout for credits/billing in AI Gateway Guide

Author: parteeksingh24

content/Guides/ai-gateway.mdx

@@ -21,6 +21,10 @@ The AI Gateway supports the following providers out of the box:
 - **Mistral**
 - **Perplexity**
 
+<Callout type="info">
+[Purchase credits and manage billing](https://app.agentuity.com/billing) in the Cloud Console.
+</Callout>
+
 ## How It Works
 
 ### Automatic Detection