API: Provide forum info under /

Closes #875.

src/Api/routes.php

@@ -16,7 +16,7 @@ use Flarum\Http\RouteHandlerFactory;
 return function (RouteCollection $map, RouteHandlerFactory $route) {
     // Get forum information
     $map->get(
-        '/forum',
+        '/',
         'forum.show',
         $route->toController(Controller\ShowForumController::class)
     );