Add new information-design-principles content

This will replace the existing 'writing-government-services' resource.
Delete old version of the content
Remove content designers

Probably aren't the primary audience for this content.
Update title
Link to new section
Remove content-designers audience

As per discussion with @sarahrichards
Rename resource

As per discussion with @sarahrichards.
Merge pull request #278 from alphagov/information-design-principles

Information design principles