add skeleton for groups controller

website/src/controllers/api-v3/groups.js

@@ -0,0 +1,20 @@
+let api = {};
+
+/**
+ * @api {get} /groups/:groupId Get a group by its id
+ * @apiVersion 3.0.0
+ * @apiName GetGroup
+ * @apiGroup Group
+ *
+ * @apiSuccess {Object} group The group object
+ */
+api.getGroup = {
+  method: 'GET',
+  url: '/groups/:groupId',
+  middlewares: [authWithHeaders()],
+  handler (req, res, next) {
+    // ...
+  },
+};
+
+export default api;