Update docs for Nextjs 13 App folder routes preview

[graemec23]

Hello I was wondering if the docs could be please updated for NextJS 13 App folder routes as its a slightly different to the examples given.

Im using this as my preview code, I'm currently gettings Cors issues but am not sure the best approach to adding the headers.

import { draftMode } from 'next/headers';
import { redirect } from 'next/navigation';

export async function GET() {
  draftMode().enable();
  redirect(`/`);
}