code: @Override
      protected Content getNavLinkNext() {
          if (next == null) {
              return getNavLinkNext(null);
          } else {
              DocPath path = DocPath.relativePath(packageElement, next);
              return getNavLinkNext(path.resolve(DocPaths.PACKAGE_TREE));
          }
      }
docs: Get link for the next package tree file.
  
  @return a content tree for the link