docs: add missing step in product builder guide (#13370)

www/apps/book/public/llms-full.txt

@@ -68634,6 +68634,18 @@ After that, you use the [addToCartWorkflow](https://docs.medusajs.com/references
 Next, you need to add the complementary and addon product variants to the cart. Replace the `TODO` with the following:
 
 ```ts title="src/workflows/add-product-builder-to-cart.ts" highlights={addProductBuilderToCartWorkflowHighlights3}
+// Step 4: Get cart after main product is added
+const { data: cartWithMainProduct } = useQueryGraphStep({
+  entity: "cart",
+  fields: ["*", "items.*"],
+  filters: {
+    id: input.cart_id,
+  },
+  options: {
+    throwIfKeyNotFound: true,
+  },
+})
+
 // Step 5: Add complementary and addon products
 const {
   items_to_add: moreItemsToAdd,