What is jazz?  The question has been debated often, but now we know how Joni Mitchell might answer.  On September 5, the legendary singer-songwriter’s Archives series continues with its first themed compilation, appropriately entitled Joni’s Jazz.  These 61 selections – available on 4 CDs or 8 LPs – draw from nearly every core album in the Mitchell discography with just a couple of exceptions (Clouds, Ladies of the Canyon) and reflect the breadth of the artist’s immersion into the jazz idiom in all its many varied forms. Last year, in her liner…