code: protected Content getNavLinkTree() {
          List<PackageElement> packages = new ArrayList<>(configuration.getSpecifiedPackageElements());
          DocPath docPath = packages.size() == 1 && configuration.getSpecifiedTypeElements().isEmpty()
                  ? pathString(packages.get(0), DocPaths.PACKAGE_TREE)
                  : pathToRoot.resolve(DocPaths.OVERVIEW_TREE);
          return HtmlTree.LI(getHyperLink(docPath, contents.treeLabel, "", ""));
      }
docs: Get "Tree" link in the navigation bar. If there is only one package
  specified on the command line, then the "Tree" link will be to the
  only "package-tree.html" file otherwise it will be to the
  "overview-tree.html" file.
  
  @return a content tree for the link