Docs: add warning about removing CNAME record from DNS after removing a domain

[stsewd]

What's the problem this feature will solve?

To completely detach a custom domain from our platform, in addition to remove it, a user also needs to remove the CNAME from their DNS. Otherwise, any user may be able to link that domain to another project (subdomain hijacking).

Describe the solution you'd like

The proper solution is to use a unique hashed domain as the CNAME (#4521, https://github.com/readthedocs/readthedocs-ext/issues/549). But we should at least mention it in our docs until we implement that.

Mention this at
https://docs.readthedocs.io/en/stable/guides/custom-domains.html#removing-a-custom-domain

Additional context

Add any other context, links, etc. relevant to the feature request.