docs: add billing alerts screenshot to the billing limits page

Author: kathiekiwi

docs/billing-limits.mdx

@@ -31,6 +31,8 @@ Billing alerts are email notifications sent when your monthly spend crosses a th
 - **With a billing limit set**: thresholds are percentages of your limit (e.g. 50%, 80%).
 - **Without a billing limit**: thresholds are dollar amounts.
 
+![Billing alerts settings](./images/billing-alerts-docs.png)
+
 Alerts only notify you — they never pause environments or reject runs. Use them on their own for visibility, or alongside a limit to get advance warning before enforcement kicks in.
 
 ## What happens when you reach your limit