chore: disable create board

frontend/app_flowy/lib/plugins/board/board.dart

@@ -31,7 +31,7 @@ class BoardPluginBuilder implements PluginBuilder {
 
 class BoardPluginConfig implements PluginConfig {
   @override
-  bool get creatable => true;
+  bool get creatable => false;
 }
 
 class BoardPlugin extends Plugin {