It takes great courage for a TV news executive to set aside an entire evening of prime time for a documentary - and ABC's Roone Arledge must be saluted for that bold move. It may not pay off in ratings (opposite ''Dallas,'' ''Falcon Crest,'' and a new Angie Dickinson show), but it indicates a dedication to public-service programming which is becoming all too scarce on commercial broadcasting.''FDR'' (ABC, Friday, 8-11 p.m.), hosted by Bob Trout and David Brinkley, is a three-hour marathon of fascinating information and entertainment, filled with what for some will be 1930s and '40s nostalgia. All four living former presidents appear on camera to comment on FDR. For many youngsters it may even be a curious history of another time long ago which they have only heard about vaguely. But for both groups it will be three hours well spent, three hours of vibrant history - helpful to learn about the country yesterday and today.''FDR,'' produced under the aegis of Pamela Hill and Richard Richter, is filled with historical news clips, interspersed with contemporary interviews with experts. It is ABC's electronic ''Fireside Chat.''