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/vercel.json

@@ -484,6 +484,10 @@
     {
       "source": "/user-guide/price-lists/import",
       "destination": "/user-guide/price-lists"
+    },
+    {
+      "source": "/development/endpoints/:path",
+      "destination": "/development/api-routes/:path"
     }
   ],
   "framework": "docusaurus-2",