Add link to hostname API route on About page

sections/About.svelte

@@ -42,6 +42,8 @@
     {/await}
   </ul>
 
+  <p>Your hostname is available in JSON format from the <a href={link('/hostname')}>/hostname</a></p>
+
   <h3>Your details</h3>
 
   <p>You are <strong>{$signedIn ? 'signed in' : 'not signed in'}.</strong></p>