Keira Knightley: ‘Early motherhood is definitely more exhausting than shooting films!’

She’s played a Jane Austen hero, a swashbuckling aristocrat, and a wartime cryptanalyst. Now, the beloved actor steps into her most unexpected role yet, as a children’s author. She talks to Jessie Thompson about her daughter’s teething problems, getting equal pay, and that famous ‘Vanity Fair’ photoshoot