Update links to the GOV.UK style guide

The GOV.UK style guide is now part of the new [content guidance](https://insidegovuk.blog.gov.uk/2014/10/07/content-guidance-now-in-one-place/).

The mappings have been done by the Depts & Policy content team.
I think https://www.gov.uk/design-principles/style-guide should point at https://www.gov.uk/guidance/content-design/writing-for-gov-uk