Steven Cantor’s Dancer plays like a PBS installment of Behind the Music with ballet standing in for rock and roll—and that’s not necessarily a bad thing. Like Mikhail Baryshnikov, Ukraine-born Sergei Polunin has an angular face, crystalline eyes, and the awe-inspiring ability to fly through the air with ease. His mother, Galina, claims he was so flexible from birth that it alarmed her nurse. He used that skill as a gymnast before switching to ballet…