Add new release folder structure (#5922)

_data/sidebars/releases_sidebar.yml

@@ -25,6 +25,8 @@ entries:
       url: /releases/latest/ios.html
   - title: 2023 Releases
     folderitems:
+    - title: April
+      url: /releases/2023-04/index.html
     - title: March
       url: /releases/2023-03/index.html
     - title: February