added link to REST API reference docs

Author: HollyCl

articles/operator-5g-core/toc.yml

@@ -54,4 +54,8 @@ items:
   - name: Monitor the deployment status
     href: how-to-monitor-deployment-status.md
   - name: Perform health and configuration checks post-deployment
-    href: how-to-perform-checks-post-deployment.md
+    href: how-to-perform-checks-post-deployment.md
+- name: Reference
+  items:
+  - name: REST API Reference
+    href: /rest/api/mobilepacketcore