docs: function copyAssets(output, plugin) {
      var logger = output.getLogger();
      var pluginRoot = plugin.getPath();
      var options = output.getOptions();
  
      var outputRoot = options.get('root');
      var assetOutputFolder = path.join(outputRoot, 'gitbook');
      var prefix = options.get('prefix');
  
      var assetFolder = path.join(pluginRoot, ASSET_FOLDER, prefix);
  
      if (!fs.existsSync(assetFolder)) {
          return Promise();
      }
  
      logger.debug.ln('copy assets from theme', assetFolder);
      return fs.copyDir(
          assetFolder,
          assetOutputFolder,
          {
              deleteFirst: false,
              overwrite: true,
              confirm: true
          }
      );
  }