docs: add manage promotions in cart storefront guide (#13483)

* docs: add manage promotions in cart storefront guide

* fix incorrect cURL example

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

@@ -74126,9 +74126,9 @@ const StripeSavedPaymentMethodsContainer = ({
     SavedPaymentMethod[]
   >([])
   const [selectedPaymentMethod, setSelectedPaymentMethod] = useState<
-    string | undefined
+    string | null
   >(
-    paymentSession?.data?.payment_method_id as string | undefined
+    paymentSession?.data?.payment_method as string | null
   )
 
   useEffect(() => {
@@ -74274,7 +74274,7 @@ Then, add a new state variable that keeps track of whether the customer is using
 
 ```ts title="src/modules/checkout/components/payment-container/index.tsx" badgeLabel="Storefront" badgeColor="blue"
 const [isUsingSavedPaymentMethod, setIsUsingSavedPaymentMethod] = useState(
-  paymentSession?.data?.payment_method_id !== undefined
+  paymentSession?.data?.payment_method !== null
 )
 ```
 
@@ -74411,7 +74411,7 @@ So, you need to update the `confirmCardPayment` usage to support passing the ID
 In `src/modules/checkout/components/payment-button/index.tsx`, find the `handlePayment` method and update its first `if` condition:
 
 ```ts title="src/modules/checkout/components/payment-button/index.tsx" badgeLabel="Storefront" badgeColor="blue"
-if (!stripe || !elements || (!card && !session?.data.payment_method_id) || !cart) {
+if (!stripe || !elements || (!card && !session?.data.payment_method) || !cart) {
   setSubmitting(false)
   return
 }
@@ -74424,7 +74424,7 @@ Then, find the usage of `confirmCardPayment` in the `handlePayment` function and
 ```ts title="src/modules/checkout/components/payment-button/index.tsx" badgeLabel="Storefront" badgeColor="blue" highlights={confirmPaymentHighlights}
 await stripe
 .confirmCardPayment(session?.data.client_secret as string, {
-  payment_method: session?.data.payment_method_id as string || {
+  payment_method: session?.data.payment_method as string || {
     card: card!,
     billing_details: {
       name: