docs(whats-new): add Week of April 13, 2026 entry

Announces Monitoring GA with links to the quickstart docs and
announcement blog post.

https://claude.ai/code/session_01LZKc1zrMXuAC3GXiGc4Gin

Author: claude

fern/changelog/2026-04-13.mdx

@@ -0,0 +1,6 @@
+# What's New: Week of April 13, 2026
+
+1. **Monitoring — GA**: Automated call quality monitoring is now generally available. Detect issues with trigger-based rules, get alerts when something goes wrong, and surface resolution suggestions — all from the dashboard.
+
+    - [Monitoring quickstart](https://docs.vapi.ai/observability/monitoring-quickstart)
+    - [Announcement blog post](https://blog.vapi.ai/monitoring)