chore: next.js dir clarification (#2269)

* Update 16_nextjs_app_router.mdx

* chore: fix style

---------

Co-authored-by: vinckr <[email protected]>

Author: Milk-Tea

docs/getting-started/integrate-auth/16_nextjs_app_router.mdx

@@ -19,6 +19,13 @@ The code used in the following quickstart is available in the
 
 ## Prerequisites
 
+:::note
+
+This quickstart assumes you're using the alias configuration and not using a `src/` directory. If your project uses `src/`, place
+all files referenced as being in the `root` directory inside `src/` instead.
+
+:::
+
 1. Create a new Next.js application using your preferred package manager. You can use the following commands to create a new
    Next.js app: