Jimmy Page, The Edge and Jack White of The White Stripes are featured in a new documentary that tells the story of the electric guitar. The three guitarists were chosen for their impact across three different generations. They each discuss and demonstrate how they adapted the guitar to their own unique styles.      It Might Get Loud, directed by Davis Guggenheim (An Inconvenient Truth), will premiere at the Cannes Film Festival in France, which gets under way next Wednesday.