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/homepage.mdx

@@ -160,6 +160,15 @@ Medusa provides the essential building blocks that developers can put together t
 Learn about all the new features and enhancements in Medusa.
 
 <DocCardList colSize={4} items={[
+  {
+    type: 'link',
+    href: '/development/api-routes/create',
+    label: 'API Routes',
+    customProps: {
+      icon: Icons['computer-desktop-solid'],
+      description: 'Create API Routes and middlewares with an improved developer experience.',
+    }
+  },
   {
     type: 'link',
     href: 'https://docs.medusajs.com/ui',
@@ -182,19 +191,6 @@ Learn about all the new features and enhancements in Medusa.
       }
     }
   },
-  {
-    type: 'link',
-    href: '/admin/routes',
-    label: 'Admin UI Routes',
-    customProps: {
-      icon: Icons['computer-desktop-solid'],
-      description: 'Learn how you can customize the admin dashboard with admin UI routes.',
-      badge: {
-        variant: 'orange',
-        children: 'Beta'
-      }
-    }
-  },
 ]} />
 
 <!-- vale docs.HeadingPunctuation = NO -->