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

Author: CoderDeltaLAN

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

@@ -48,6 +48,17 @@ export const config = {
 
 ## Exports
 
+> **Note**
+> Place `middleware.{js,ts}` at the **project root** or **`/src` root**:
+>
+> ```
+> /middleware.ts        ✅
+> /src/middleware.ts    ✅
+> /app/middleware.ts    ❌ (not supported)
+> ```
+>
+> If the file is not at root, Next.js will not load it. See also: [Matcher config](https://nextjs.org/docs/app/building-your-application/routing/middleware#matcher).
+
 ### Middleware function
 
 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.