NEW YORK — Imagine if Bob Woodward’s clandestine meetings in a Washington, D.C., parking garage with Deep Throat had been documented — or, better yet, filmed by Woodward, himself. The analogy isn’t perfect, but that’s about the closest equivalent to Laura Poitras’ one-of-a-kind documentary “Citizenfour,” which captures former National Security Agency contractor Edward Snowden during his leak of NSA documents to Poitras (a documentarian and reporter) and journalist Glenn …