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/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