types: {
      'application/rss+xml': 'https://nextjs.org/rss',
    },
  },
}
<head> output <link rel="canonical" href="https://nextjs.org" />
<link rel="alternate" hreflang="en-US" href="https://nextjs.org/en-US" />
<link rel="alternate" hreflang="de-DE" href="https://nextjs.org/de-DE" />
<link
  rel="alternate"
  media="only screen and (max-width: 600px)"
  href="https://nextjs.org/mobile"
/>
<link
  rel="alternate"
  type="application/rss+xml"
  href="https://nextjs.org/rss"
/>
appLinks
layout.js / page.js export const metadata = {
  appLinks: {
    ios: {
      url: 'https://nextjs.org/ios',