( Editor’s Note: This preview of the SILVERDOCS film festival comes to us from Sommer Mathis, who has joined our staff to write about arts.) Just when you were certain that downtown Silver Spring will never be as hip as it wishes it was, SILVERDOCS: AFI/Discovery Channel Documentary Festival returns for its 3rd year to challenge your deeply held beliefs. This year’s opening night gala alone is enough to make any film geek perspire with…