Connect a custom domain to a site.
IMPORTANT: between this call and a working domain stands A PERSON. You
get DNS records — show them to the user and explain they go in at their
domain registrar. Do not wait for readiness: DNS propagates in minutes
to an hour, the platform re-checks on its own, and you check later via
`check_domain`. Spinning here burns time and context for nothing.
The address can be passed as-is: the platform reduces
`https://shop.example.com/page` to `shop.example.com` itself.
Connector