They’ve turned hanging out into a mission, which makes them not so far removed from the heroes of “Wayne’s World.” But the Deadheads also evince an honest yearning for something that’s tribal, primitive, sacred, ancient, shamanic, ecstatic. Watching the documentary, you can’t help but empathize with their desire to find an alternate lifestyle. But it also occurs to you that this wouldn’t be a bad setting for “Midsommar II.”