code: public static Optional<ButtonType> alert(String title, String content) {
          return alert(title, null, content);
      }
docs: 弹窗，header默认为null，alertType默认为{@link AlertType#INFORMATION}，modality默认为{@link
  Modality#NONE}，window默认为null，style默认为{@link StageStyle#DECORATED}
  
  @param title 标题
  @param content 内容
  
  @return {@link ButtonType}