Add styles for the new site wrapper
Add styles for the new site wrapper

Rather than make a breaking change to govuk-elements-sass by removing
@extend %site-width-container from the #content area, instead -
override it in the documentation styles. Not pretty, but I’d prefer not
to make a breaking change for a fix to govuk_elements (the app).

alphagov/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements