External link checking

It'd be nice if we checked external links out-of-band with the regular docs build. Like, maybe run the link checking every day and get warned about failures but don't fail the main build. I see we talked about it in #112 and mentioned doing it for the entire public site. We can certainly do that if we want but I suspect the consumers of the link check for docs isn't the same as the link check for the public site. Either way is fine with me, but it would be nice to get warnings about broken external links automatically.