Validate DNS and CDN changes required for launch

In order to know if we'll experience downtime during launch, we need to test moving a domain name from one Pages project to another.

When we take the domain off gitlab-docs and add it to docs-gitlab-com, there may be a few minutes where the site won't load as the domain is re-validated and the new LetsEncrypt certificate provisions.

If this move does have to incur downtime, we'll need to plan for a maintenance window. We can probably follow a lighter-weight version of this process: https://handbook.gitlab.com/handbook/engineering/infrastructure/change-management/#communicating-a-change-that-requires-downtime-maintenance-window

We may also be able to avoid downtime using Cloudflare (TBD, we'll need to discuss with Infra folks on this, see gitlab-com/gl-infra/production-engineering#26228 (closed)).