Create archive site for v3 documentation

[vanitabarrett]

What

For previous breaking releases, we have created an old 'archive' site on Netlify to "preserve" the design system documentation for users who might be unable to update to the breaking release and are stuck on previous versions.

Example: https://v2--govuk-design-system-preview.netlify.app

We should generate a similar site for the latest version of v3.x.x before we release v4.0.0

Why

Some users may be unable to update to v4.0.0. These users may benefit from being able to look at the documentation for the version they're currently on as the live design system site may refer to code and features which they don't have access to.

Further detail

There is an existing 'v3' Netlify domain which looks like it was created when the v3.0.0 breaking release was being worked on. That v3 branch no longer exists in Github. It doesn't seem possible to delete Netlify apps once they've been created, so we will need to create a new v3 branch which will hopefully 🤞 overwrite what's deployed to the existing v3-- netlify domain.

Who needs to know about this

Developers

Done when

• https://github.com/alphagov/design-system-team-internal/issues/332
• v3 archive site created for the design system website
• decided whether we need to do anything similar for the frontend docs site

[lfdebrux]

We should hold on this until just before (or right after) we release v4, as we might still have fixes to the 'v3' website between now and then.

[vanitabarrett]

Design System archive site: https://v3--govuk-design-system-preview.netlify.app/

[vanitabarrett]

Frontend Docs archive site: https://v3--govuk-frontend-docs-preview.netlify.app/

[vanitabarrett]

Have added these links to the support questions ideation doc too