Add section on design patterns

- Add a new section on design patterns
- Link to the alpha [GOV.UK elements](http://govuk-elements.herokuapp.com/) page
- Delete guides made obsolete by the new patterns and elements content

Redirects will need setting up from the deleted guides to the most appropriate new content. [I've specified what these should be here](https://docs.google.com/a/digital.cabinet-office.gov.uk/spreadsheets/d/1Y4tv2qhoXgONUq8QzwJNmn2oYub3v1gaY47mEDTw87M/edit?usp=sharing).

I appreciate this is a big request. Happy to sit with whoever and go through it in detail if that helps.