doc: Add a Foreword and a Preface

In particular, the preface defines the scope of this document we're building
-- is the definition there enough with respect to what we want with this?

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>