Restructure service.gov.uk guidance

We've had a lot of feedback, questions and general email about which domains should be used where, how other services hang off those domains, and so on.

This pull request breaks up our existing guidance into sections that more clearly map to the way people are thinking about domains, and adds some more explanations.

It's not intended to be entirely comprehensive, but pulls together content from quite a few documents and provides a new structure for holding it all together.