Add Kontent example to preview section (#21542)

Kontent example also contains the implementation of the preview capabilities in the example.

Including the example as it is [Data fetching section](https://nextjs.org/docs/basic-features/data-fetching).

docs/advanced-features/preview-mode.md

@@ -21,6 +21,7 @@ description: Next.js has the preview mode for statically generated pages. You ca
     <li><a href="https://github.com/vercel/next.js/tree/canary/examples/cms-buttercms">ButterCMS Example</a> (<a href="https://next-blog-buttercms.now.sh/">Demo</a>)</li>
     <li><a href="https://github.com/vercel/next.js/tree/canary/examples/cms-storyblok">Storyblok Example</a> (<a href="https://next-blog-storyblok.now.sh/">Demo</a>)</li>
     <li><a href="https://github.com/vercel/next.js/tree/canary/examples/cms-graphcms">GraphCMS Example</a> (<a href="https://next-blog-graphcms.now.sh/">Demo</a>)</li>
+    <li><a href="https://github.com/vercel/next.js/tree/canary/examples/cms-kontent">Kontent Example</a> (<a href="https://next-blog-kontent.vercel.app//">Demo</a>)</li>
   </ul>
 </details>