docs: update endpoints to use file-routing approach (#5397)

- Move the original guides for creating endpoints and middlewares to sub-sections in the Endpoints category.
- Replace existing guides for endpoints and middlewares with the new approach.
- Update all endpoints-related snippets across docs to use this new approach.

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

@@ -8,7 +8,7 @@ info:
 tags:
   - name: Auth
     description: >
-      Authentication endpoints allow customers to manage their session, such as
+      Authentication API Routes allow customers to manage their session, such as
       login or log out.
 
       When a customer is logged in, the cookie header is set indicating the
@@ -40,8 +40,8 @@ tags:
       Customers can use gift cards during checkout to deduct the gift card's
       balance from the checkout total.
 
-      The Gift Card endpoints allow retrieving a gift card's details by its
-      code. A gift card can be applied to a cart using the Carts endpoints.
+      The Gift Card API Routes allow retrieving a gift card's details by its
+      code. A gift card can be applied to a cart using the Carts API Routes.
     externalDocs:
       description: How to use gift cards in a storefront
       url: https://docs.medusajs.com/modules/gift-cards/storefront/use-gift-cards
@@ -49,8 +49,8 @@ tags:
     description: >
       Orders are purchases made by customers, typically through a storefront.
 
-      Orders are placed and created using the Carts endpoints. The Orders
-      endpoints allow retrieving and claiming orders.
+      Orders are placed and created using the Carts API Routes. The Orders
+      API Routes allow retrieving and claiming orders.
     externalDocs:
       description: How to retrieve order details in a storefront
       url: >-
@@ -75,7 +75,7 @@ tags:
       cards](https://docs.medusajs.com/modules/gift-cards/storefront/use-gift-cards)
       in a store.
 
-      Using these endpoints, you can filter products by categories, collections,
+      Using these API Routes, you can filter products by categories, collections,
       sales channels, and more.
     externalDocs:
       description: How to show products in a storefront
@@ -89,7 +89,7 @@ tags:
       Products can be categoriezed into categories. A product can be associated
       more than one category.
 
-      Using these endpoints, you can list or retrieve a category's details and
+      Using these API Routes, you can list or retrieve a category's details and
       products.
     externalDocs:
       description: How to use product categories in a storefront
@@ -100,7 +100,7 @@ tags:
       such as marketing or discount purposes. For example, you can create a
       Summer Collection.
 
-      Using these endpoints, you can list or retrieve a collection's details and
+      Using these API Routes, you can list or retrieve a collection's details and
       products.
   - name: Product Tags
     description: |