LOS ANGELES — After weeks of negotiations and intrigue, Doc Rivers has officially left the Boston Celtics for the Los Angeles Clippers.Rivers will be introduced as the Clippers’ new coach and senior vice president of basketball operations at a news conference today at their Playa Vista training complex, capping a lengthy process by completing a rare trade involving a championship-winning coach.The Clippers and Celtics finalized the move Tuesday when the NBA approved the deal. Boston will get an unprotected first-round pick in 2015 from the Clippers for Rivers, who went 416-305 and won the 2008 NBA title during nine seasons with the Celtics.Boston’s front office mostly had warm words for Rivers after he took off for his exciting new team on the West Coast, apparently not eager to stick around for the aging Celtics’ rebuilding process.“We don’t have a championship without Doc Rivers coaching,” said Danny Ainge, the Celtics’ president of basketball operations. “He did an unbelievable job.