code: protected Content getNavLinkClassIndex() {
          Content allClassesContent = getHyperLink(pathToRoot.resolve(
                  DocPaths.ALLCLASSES_NOFRAME),
                  allclassesLabel, "", "");
          Content li = HtmlTree.LI(allClassesContent);
          return li;
      }
docs: Get link for generated index. If the user has used "-splitindex"
  command line option, then link to file "index-files/index-1.html" is
  generated otherwise link to file "index-all.html" is generated.
  
  @return a content tree for the link