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/docs/content/development/entities/migrations/overview.mdx

@@ -80,10 +80,10 @@ Developers can create custom entities in the Medusa backend, a plugin, or in a m
   {
     type: 'link',
     href: '/development/entities/create',
-    label: 'Create an Endpoint',
+    label: 'Create an API Route',
     customProps: {
       icon: Icons['academic-cap-solid'],
-      description: 'Learn how to create endpoints in Medusa.'
+      description: 'Learn how to create API Routes in Medusa.'
     }
   },
 ]} />