By Nicola Methven A new BBC documentary aims to “demystify” the death of Nirvana’sKurt Cobain by telling the shocking story of the tragedy and its aftermath in real time, using archive footage. BBC2 is to mark the 30th anniversary of Cobain’s death in 1994 with a dedicated night of programming next month. The highlight will be the hour-long documentary Moments That Shook Music: Kurt Cobain, which will include some footage never shown before on British TV. In 1994, Nirvana had achieved global success as one of the biggest bands on the planet. But suddenly their lead singer was gone, having take…