Think of it as the Marché du Film, except just for documentaries. That’s one way to picture Sunny Side of the Doc, the annual gathering of some of the top buyers and sellers of nonfiction content around the world. The four-day event got underway Monday in La Rochelle, the historic port city on France’s Atlantic coast. Much like the Marché du Film in Cannes, Sunny Side’s exhibition hall presents a maze of booths where countries and companies alike do business along the water: Arte, Canal+, Mediawan, NHK, Terra Mater Factual Studios, Newen Connect, Al Jazeera, National Geographic, PBS, BBC, and...