Give the service handbook a sub-domain

[stevenjmesser]

What

The service handbook is viewable at https://digital-land.github.io/service-handbook/ but we want to be able to view it at https://handbook.planning.data.gov.uk

Why

Given recent problems with non-GOV.‌UK domains (resolved by digital-land/digital-land#352), hosting the service handbook on a GOV.‌UK domain will ensure staff can always access it.

Background info

The service handbook is similar to technical documentation in that it's an Eleventy site built using GitHub Actions and hosted on GitHub Pages. We can configure GitHub Pages to use our DNS and the sub-domain we create in Route 53.

[stevenjmesser]

Assigning to @Ben-Hodgkiss for refinement and planning.