update homepage and delete showcase that gets 404

Author: kfahn22

content/pages/homepage/index.json

@@ -54,7 +54,7 @@
       "challenges/127-brownian-tree-snowflake/showcase/contribution7.json",
       "challenges/138-angry-birds-with-matterjs/showcase/contribution2.json",
       "challenges/89-langtons-ant/showcase/contribution5.json",
-      ""
+      "challenges/98-quadtree/showcase/contribution4.json"
     ],
     "showcaseCta": {
       "text": "Want to see more?",