Set in the Black Hills of South Dakota, Mount Rushmore may be one of the United States’ most recognizable landmarks, but as author Matthew Davis explains in “A Biography of a Mountain – The Making and Meaning of Mount Rushmore” (St. Martin’s Press), there’s a lot that most Americans don’t realize about it. “I don’t think that your average American knows who created Mount Rushmore,” Davis told The Post. State historian Doane Robinson commissioned sculptor Gutzon Borglum to create the monument in