added documentation for entities

www/docs/sidebars.js

@@ -143,6 +143,11 @@ module.exports = {
                 },
               ]
             },
+            {
+              type: "doc",
+              id: "advanced/backend/entities",
+              label: "Entities"
+            },
             {
               type: "category",
               label: 'Shipping',