The yearly visits by the Mariinsky Ballet — as in their Sleeping Beauty in 2010 and Don Quixote in 2009 — are generally one of the highlights of the Kennedy Center’s dance season, and this is certainly true of this week’s production of the St. Petersburg company’s classic Giselle. It is a choreography and staging that are instantly recognizable as the best that the classical ballet tradition has to offer — especially the ballet blanc of the second act, pictured at right — giving the spectator a sort of mythic image of what ballet is.