Nearby, in the postcard-pretty town of Stockbridge — birthplace of Arlo Guthrie’s “Alice’s Restaurant” — the Norman Rockwell Museum is marking its centenary of the artist’s first Saturday Evening Post cover. From 1916’s “Boy with Baby Carriage” comes “Build a Better Baby Carriage,” a kicky show of contemporary sculpture on its lush grounds. Then duck inside “the room where it happened”: Rockwell’s own, perfectly preserved studio.