docs: added override cart completion strategy (#3976)

* docs: added override cart completion strategy

* fixed eslint errors

* small fixes

* fixed eslint error

www/docs/sidebars.js

@@ -684,6 +684,11 @@ module.exports = {
           id: "modules/carts-and-checkout/backend/add-payment-provider",
           label: "Backend: Create Payment Processor",
         },
+        {
+          type: "doc",
+          id: "modules/carts-and-checkout/backend/cart-completion-strategy",
+          label: "Backend: Override Cart Completion",
+        },
         {
           type: "doc",
           id: "modules/carts-and-checkout/storefront/implement-cart",