Update the Frontend Docs to use GOV.UK Frontend v4.0.0

[vanitabarrett]

What

Once GOV.UK Frontend 4.0.0 has been released to npm, update the Prototype Kit to use it. As this is a breaking release, we might want to generate a pre-release ahead of the actual release, so we can test updating to v4.0.0 in the Prototype Kit and prepare any changes we need to make in advance.

Why

So that the Frontend Docs and Sass documentation reflects the latest release of GOV.UK Frontend

Done When

• Frontend Docs updated to use GOV.UK Frontend v4.0.0
• Sass docs updated (e.g: govuk-main-wrapper mixins are no longer documented)