One‐Pot Domino Synthesis of Diarylalkynes/1,4‐Diaryl‐1,3‐diynes by [9,9‐Dimethyl‐4,5‐bis(diphenylphosphino)xanthene] (Xantphos)–Copper(I) Iodide–Palladium(II) Acetate‐Catalyzed Double Sonogashira‐Type Reaction