Update package api docs and changelog

changelogs/unreleased/35527-add-created-at-to-package.yml

+---
+title: Adds created_at object to package api response
+merge_request: 20816
+author:
+type: added

doc/api/packages.md

@@ -31,13 +31,15 @@ Example response:
     "id": 1,
     "name": "com/mycompany/my-app",
     "version": "1.0-SNAPSHOT",
-    "package_type": "maven"
+    "package_type": "maven",
+    "created_at": "2019-11-27T03:37:38.711Z"
   },
   {
     "id": 2,
     "name": "@foo/bar",
     "version": "1.0.3",
-    "package_type": "npm"
+    "package_type": "npm",
+    "created_at": "2019-11-27T03:37:38.711Z"
   }
 ]
 ```