$arResult['ITEMS_BY_GROUP'][$arSection['NAME']][$arResult['ITEMS'][$key]['NAME']] =
$arResult['ITEMS'][$key]['DETAIL_PAGE_URL'];
<dl class="aside-menu">
<?foreach($arResult['ITEMS_BY_GROUP'] as $group_name=>$elements):?>
<dt><?=$group_name?></dt>
<dd>
<?foreach($elements as $name=>$href):?>
<span>
<a href="<?=$href?>"><?=$name?></a>
</span>
<?endforeach;?>
</dd>
<?endforeach;?>
</dl>