def target_dir( f = nil )
      if File.exists?( config_file_path )
        form_file_name( target_dir_from_config, f )
      else
        form_file_name( File.join( ::Rails.root.to_s, 'public/docs' ), f )
      end
    end
def target_dir( f = nil )
      if File.exists?( config_file_path )
        form_file_name( target_dir_from_config, f )
      else
        form_file_name( File.join( ::Rails.root.to_s, 'public/docs' ), f )
      end
    end
return the directory where rapidoc generates the doc