Add new release folder structure (#2831)

_data/sidebars/releases_sidebar.yml

@@ -25,6 +25,8 @@ entries:
       url: /releases/latest/go.html
   - title: 2021 Releases
     folderitems:
+    - title: May
+      url: /releases/2021-05/index.html
     - title: April
       url: /releases/2021-04/index.html
     - title: March