Well, why not? It's a great host with a whole bunch of features you'd have to invent yourself otherwise like feature branch previews. It's a static site/documentation site afterall.
In this case to a proper government domain... aha, I bet there's some policy against pointing .gov.uk hostnames to a third-party platform?
Some guidance that might be relevant: https://www.gov.uk/guidance/use-your-govuk-domain-name?step-...
I agree it looks cheesy and makes me wonder if it's really official government documetnation, without a .gov.uk hostname (and SSL cert, naturally).
Am I misunderstanding something?