Add distinct page titles

Should have better SEO properties in terms of being easier to understand what
each page is about in SERPs.

https://www.google.co.uk/search?q=gov+uk has a first result of 'Welcome to GOV.UK'. This is the html page <title/> element

https://www.google.co.uk/search?q=gov+uk+style+guide has a first result of 'Style Guide - Gov.UK'. This is not the html page <title/> element, which is in fact <title>GDS design principles</title>

If we fix the title, we may have the correct search results*.