Show `show_doc` info in preview window along with link to full docs.

def doc(elt):
    "Show `show_doc` info in preview window along with link to full docs."
    global use_relative_links
    use_relative_links = False
    elt = getattr(elt, '__func__', elt)
    md = show_doc(elt, markdown=False)
    if is_fastai_class(elt):
        md += f'\n\n<a href="{get_fn_link(elt)}" target="_blank" rel="noreferrer noopener">Show in docs</a>'
    output = HTMLExporter().markdown2html(md)
    use_relative_links = True
    if IS_IN_COLAB: get_ipython().run_cell_magic(u'html', u'', output)
    else:
        try: page.page({'text/html': output})
        except: display(Markdown(md))