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