Disney’s Frozen 2 is currently the biggest animated film of all time. And it’s easy to suspect that something so widely and instantaneously accepted, the world over, appeared one day, fully formed and ready for box office and pop-cultural dominance. But that’s not how it works. Making any Disney animated feature is a years-long process full of ups and down, snowy peaks and icy valleys, and all of that drama is captured exquisitely in the new Disney+ documentary series Into the Unknown: Making Frozen 2. (All six episodes debut this week.) Through six episodes, you …