Link to quick intro from main help page (#405)

* Demo page

* Add link from main help to demo page

* Make it correct

* remove spurious file

packages/frontend/src/help/index.mdx

@@ -11,6 +11,8 @@ import { Warning } from "../components/alert";
 - a technology created by [Topos Institute](https://topos.site/) and our
   partners: <A href="credits">read the credits</A>
 
+You can find a page with some examples of CatColab in action <A href="quick-intro">here</A>.
+
 <Warning>
 CatColab is under active development with new features added regularly. We aim
 to preserve the integrity of your data but you should not store anything