“Skywalkers: A Love Story” joins “Free Solo” in a film subgenre that can only be described as “F*ck That.” It’s an ultra-specific genre of documentary that showcases feats that are so incredibly dangerous that you not only shake your head in disbelief but actually get your heart beating harder as you hold your breath, imagining that if you don’t make a noise, then maybe these people won’t die right before your eyes.