Add annotations for pages 1-3 in annotations.json

Added three new annotation objects to `annotations.json`.
Each annotation includes an `id`, `page`, `x`, and `y` property.
The annotations are for pages 1, 2, and 3, all with the same
coordinates (390.0, 980.0).

EnvelopeGenerator.ReceiverUI/wwwroot/fake-data/annotations.json

@@ -0,0 +1,20 @@
+[
+  {
+    "id": 1,
+    "page": 1,
+    "x": 390.0,
+    "y": 980.0
+  },
+  {
+    "id": 2,
+    "page": 2,
+    "x": 390.0,
+    "y": 980.0
+  },
+  {
+    "id": 3,
+    "page": 3,
+    "x": 390.0,
+    "y": 980.0
+  }
+]