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);
          Content liNavField = new HtmlTree(HtmlTag.LI);
          addNavSummaryLink(memberSummaryBuilder,
                  "doclet.navField",
                  VisibleMemberMap.Kind.ANNOTATION_TYPE_FIELDS, liNavField);
          addNavGap(liNavField);
          ulNav.addContent(liNavField);
          Content liNavReq = new HtmlTree(HtmlTag.LI);
          addNavSummaryLink(memberSummaryBuilder,
                  "doclet.navAnnotationTypeRequiredMember",
                  VisibleMemberMap.Kind.ANNOTATION_TYPE_MEMBER_REQUIRED, liNavReq);
          addNavGap(liNavReq);
          ulNav.addContent(liNavReq);
          Content liNavOpt = new HtmlTree(HtmlTag.LI);
          addNavSummaryLink(memberSummaryBuilder,
                  "doclet.navAnnotationTypeOptionalMember",
                  VisibleMemberMap.Kind.ANNOTATION_TYPE_MEMBER_OPTIONAL, liNavOpt);
          ulNav.addContent(liNavOpt);
          return ulNav;
      }
docs: Get summary links for navigation bar.
  
  @return the content tree for the navigation summary links