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

* docs: add manage promotions in cart storefront guide

* fix incorrect cURL example

www/apps/api-reference/specs/store/code_samples/Shell/store_carts_{id}_promotions/delete.sh

@@ -1,2 +1,6 @@
 curl -X DELETE '{backend_url}/store/carts/{id}/promotions' \
--H 'x-publishable-api-key: {your_publishable_api_key}'
+-H 'x-publishable-api-key: {your_publishable_api_key}' \
+-H 'Content-Type: application/json' \
+--data-raw '{
+  "promo_codes": ["{value}"]
+}'

www/apps/api-reference/specs/store/openapi.full.yaml

@@ -1863,6 +1863,9 @@ paths:
       x-sidebar-summary: Add Promotions
       description: Add a list of promotions to a cart.
       x-authenticated: false
+      externalDocs:
+        url: https://docs.medusajs.com/v2/resources/storefront-development/cart/manage-promotions#add-promotion-to-cart
+        description: 'Storefront guide: How to add a promotion to cart.'
       parameters:
         - name: id
           in: path
@@ -1933,6 +1936,9 @@ paths:
       summary: Remove Promotions from Cart
       description: Remove a list of promotions from a cart.
       x-authenticated: false
+      externalDocs:
+        url: https://docs.medusajs.com/v2/resources/storefront-development/cart/manage-promotions#remove-promotion-from-cart
+        description: 'Storefront guide: How to remove a promotion from cart.'
       parameters:
         - name: id
           in: path
@@ -1963,7 +1969,11 @@ paths:
           label: cURL
           source: |-
             curl -X DELETE '{backend_url}/store/carts/{id}/promotions' \
-            -H 'x-publishable-api-key: {your_publishable_api_key}'
+            -H 'x-publishable-api-key: {your_publishable_api_key}' \
+            -H 'Content-Type: application/json' \
+            --data-raw '{
+              "promo_codes": ["{value}"]
+            }'
       tags:
         - Carts
       responses:

www/apps/api-reference/specs/store/paths/store_carts_{id}_promotions.yaml

@@ -4,6 +4,10 @@ post:
   x-sidebar-summary: Add Promotions
   description: Add a list of promotions to a cart.
   x-authenticated: false
+  externalDocs:
+    url: >-
+      https://docs.medusajs.com/v2/resources/storefront-development/cart/manage-promotions#add-promotion-to-cart
+    description: 'Storefront guide: How to add a promotion to cart.'
   parameters:
     - name: id
       in: path
@@ -75,6 +79,10 @@ delete:
   summary: Remove Promotions from Cart
   description: Remove a list of promotions from a cart.
   x-authenticated: false
+  externalDocs:
+    url: >-
+      https://docs.medusajs.com/v2/resources/storefront-development/cart/manage-promotions#remove-promotion-from-cart
+    description: 'Storefront guide: How to remove a promotion from cart.'
   parameters:
     - name: id
       in: path