Update Whitehall style guide advice
Update Whitehall style guide advice

* Link to guidance about 2.3.2, especially the fluid grid

robinwhittleton/whitehall,askl56/whitehall,ggoral/whitehall,ggoral/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,alphagov/whitehall,robinwhittleton/whitehall,alphagov/whitehall,askl56/whitehall,YOTOV-LIMITED/whitehall,ggoral/whitehall,askl56/whitehall,hotvulcan/whitehall,hotvulcan/whitehall,ggoral/whitehall,alphagov/whitehall,hotvulcan/whitehall,askl56/whitehall,robinwhittleton/whitehall,YOTOV-LIMITED/whitehall,YOTOV-LIMITED/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall