docs: add how-to guides for retrieving cart and order totals (#13044)

www/apps/resources/sidebars/order-module.mjs

@@ -82,6 +82,13 @@ export const orderSidebar = [
         sort_sidebar: "alphabetize",
         description:
           "Learn how to use the Order Module in your customizations on the Medusa application server.",
+        children: [
+          {
+            type: "link",
+            path: "/commerce-modules/order/order-totals",
+            title: "Retrieve Order Totals",
+          },
+        ],
       },
       {
         type: "category",