President-elect Donald J. Trump, who has made few public appearances since Election Day, returned on Saturday to Madison Square Garden, the site of an inflammatory campaign rally late last month, to face a warm welcome from tens of thousands of people at an Ultimate Fighting Championship event.Mr. Trump walked in to “American Bad Ass,” by Kid Rock, and was met with a chant of “U.S.A.! U.S.A.!”As the president-elect took his seat around 10 p.m. at the edge of the octagonal fighters’ cage, the arena for a few minutes could have been mistaken for one of his rallies. In an unusually...