code: protected Content getNavSummaryLinks() {
          Content li = HtmlTree.LI(contents.summaryLabel);
          li.addContent(Contents.SPACE);
          Content ulNav = HtmlTree.UL(HtmlStyle.subNavList, li);
          MemberSummaryBuilder memberSummaryBuilder = (MemberSummaryBuilder)
                  configuration.getBuilderFactory().getMemberSummaryBuilder(this);
          for (VisibleMemberMap.Kind kind : VisibleMemberMap.Kind.summarySet) {
              Content liNav = new HtmlTree(HtmlTag.LI);
              if (kind == VisibleMemberMap.Kind.ENUM_CONSTANTS && !utils.isEnum(typeElement)) {
                  continue;
              }
              if (kind == VisibleMemberMap.Kind.CONSTRUCTORS && utils.isEnum(typeElement)) {
                  continue;
              }
              AbstractMemberWriter writer =
                  ((AbstractMemberWriter) memberSummaryBuilder.getMemberSummaryWriter(kind));
              if (writer == null) {
                  liNav.addContent(contents.getContent(VisibleMemberMap.Kind.getNavLinkLabels(kind)));
              } else {
                  writer.addNavSummaryLink(
                          memberSummaryBuilder.members(kind),
                          memberSummaryBuilder.getVisibleMemberMap(kind), liNav);
              }
              if (kind != Kind.METHODS) {
                  addNavGap(liNav);
              }
              ulNav.addContent(liNav);
          }
          return ulNav;
      }
docs: Get summary links for navigation bar.
  
  @return the content tree for the navigation summary links