Review and change structure of document site

[vickytnz]

The old document site is very much based on the NHS Frontend install https://nhsuk.github.io/nhsuk-frontend/ which is a different use case from the prototype kit.

It makes more sense to align it with a mix of:

• the GOV.UK prototype kit https://prototype-kit.service.gov.uk/docs/
• more gov product oriented sites like NHS notify https://notify.nhs.uk/ govuk pay https://www.payments.service.gov.uk/ or govuk forms https://www.forms.service.gov.uk/

This would help:

• people understand the value of the product
• get started easily
• use other documentation as is needed
• get support if they need it