The Post’s front page hailing hometown hoops heroes the Knicks became an instant classic overnight — with fans buying out every copy in delis and newsstands across New York City. “It’s a keepsake, an archive. Something I might frame, first title in 53 years,” said Kristopher Kennedy, a 25-year-old middle-school teacher, at a Hudson News stand Sunday, his voice hoarse from a long night of celebrating in Midtown . The Knicks beat the San Antonio Spurs on Saturday, winning their fourth game in the