Add demo link for GroupTopper plugin

Author: HSGamer

src/timedtopper/index.md

@@ -12,6 +12,9 @@ hero:
     - theme: brand
       text: Download
       link: https://www.spigotmc.org/resources/timedtopper.116995/
+    - theme: alt
+      text: Demo
+      link: https://modrinth.com/plugin/grouptopper
     - theme: alt
       text: Explore
       link: ./config/