Update admin.json

Author: adamfowlerit

_data/portals/admin.json

@@ -563,6 +563,10 @@
         "portalName": "Copilot Studio",
         "primaryURL": "https://copilotstudio.microsoft.com/"
       },
+      {
+        "portalName": "Copilot Studio agent usage estimator",
+        "primaryURL": "https://microsoft.github.io/copilot-studio-estimator/"
+      },
       {
         "portalName": "Microsoft Fabric",
         "primaryURL": "https://fabric.microsoft.com/"