Wednesday, March 26 ADVENTURES AT THE JADE SEA (CBS, 7:30-8:30 p.m.)* William Holden narrates as an expedition of fellow conservationists and photographers visit the shores of Lake Rudolf in Kenya "to witness a way of life earmarked for extinction as the inev itable outcome of conflict between civ ilized and primitive man."
MARCUS WELBY, M.D. (ABC, 9-11 p.m.).
A special preview of next season's series featuring Robert Young as a dedicated family physician. James Brolin plays the doctor's young assistant; Anne Baxter, Susan Strasberg and Lew Ayres are guests.
KRAFT MUSIC HALL (NBC,...