docs: clarify middleware location (root or /src; /app not supported)

Author: CoderDeltaLAN

docs/01-app/03-api-reference/03-file-conventions/middleware.mdx

@@ -48,7 +48,8 @@ export const config = {
 
 ## Exports
 
-### Middleware function
+> **Note**  
+> Place `middleware.{js,ts}` at the project root or inside `src/` at the same level as `app` or `pages`. See: [Getting Started → Route Handlers and Middleware → Convention](https://nextjs.org/docs/app/getting-started/route-handlers-and-middleware#convention).
 
 The file must export a single function, either as a default export or named `middleware`. Note that multiple middleware from the same file are not supported.