docs: added zitadel-vanilla-js link and reference (#10807)

Added a link to the new vanilla-js SPA example on the sidebar, and the
card + image on the resources here:
- https://zitadel.com/docs/sdk-examples/introduction 

New sample app:
- https://github.com/zitadel/zitadel-vanilla-js

docs/sidebars.js

@@ -125,6 +125,11 @@ module.exports = {
           label: "Pylon",
           href: "https://github.com/getcronit/pylon",
         },
+        {
+          type: "link",
+          label: "Vanilla-JS",
+          href: "https://github.com/zitadel/zitadel-vanilla-js",
+        },
       ],
     },
     {