Add index file to navigation structure in zensical.toml

Author: DeekshithSH

zensical.toml

@@ -62,7 +62,8 @@ nav = [
      "setup/vps/workers-instance.md",
      "setup/vps/domain-dns.md",
      "setup/vps/customization.md"
-    ]}
+    ]},
+    "setup/index.md"
   ]},
   {"Customization" = [
     "customization/index.md",