Add icon for next-generation onions in the style guide
It would be useful to have an icon like the
onion-alt that symbolises that it's a "next-generation" onion service. This would allow website owners to use the
onion/onion-alt icon to link to their version 2 onion service and the
onion-ng/onion-ng-alt icon to link to their next-generation onion service (version 3).