Jean Shepherd, who has been reminiscing about his Midwest boyhood for years on New York radio, now is doing it on a national television network. "Jean Shepherd's America," on the public broadcasting lineup, had its premiere Sunday night, a half hour in which the narrator, off camera, recalled sights, sounds and experiences when at 17 he worked in a steel mill. As he talked, the screen was occupied by the operations of a contemporary steel mill. The star talked of the excitement of buying a pair of safety shoes and getting a new hard hat. He recalled a fall, dislocating both shoulders, from a stair high above the mill. Meanwhile, the viewer with a color set saw fascinating film, ribbons of molten steel flowing in designs that often looked more like art than industry. It was a novel approach - a commentary used as counterpoint rather than explanation of the pictures. And it was very interesting.