code: public Content getNavLinkNext(DocPath next) {
          Content li;
          if (next != null) {
              li = HtmlTree.LI(getHyperLink(next, nextLabel, "", ""));
          }
          else
              li = HtmlTree.LI(nextLabel);
          return li;
      }
docs: Get link for next file.  If next is null, just print the label
  without linking it anywhere.
  
  @param next File name for the next link
  @return a content tree for the link