code: protected Content getNavLinkTree() {
          Content linkContent = classdoc.containingPackage().isIncluded() ?
              getHyperLink(DocPath.parent.resolve(DocPaths.PACKAGE_TREE), treeLabel) :
              getHyperLink(pathToRoot.resolve(DocPaths.OVERVIEW_TREE), treeLabel);
          Content li = HtmlTree.LI(linkContent);
          return li;
      }
docs: Get the tree link.
  
  @return a content tree for the tree link