Revisions to published governance guidance

Iteration of governance guidance based on user feedback. Content designed by @beckstrous 

Made a previous pull request https://github.com/alphagov/government-service-design-manual/pull/465/files#r18892625 with same content (but as a fork, not branch), issues from the fork have been addressed in this branch except for one use of 'empowered'. See link for details.