Documentation: Add how to manage (edit and remove) custom domains

We have a page on Pages custom domains: https://docs.gitlab.com/ee/user/project/pages/custom_domains_ssl_tls_certification/ but that page only covers how to set it up. There isn't anything about how to edit/remove them.

Referring to:

Follow up

when completed or as part of the same MR, add how to do this for group domain verification as well in doc/user/enterprise_user/index.md:

### Manage domains in group

To edit or remove a domain:

1. When viewing [Domain Verification](#view-domains-in-group), select the project listed next to the relevant domain.
1. Edit or remove a domain following the relevant [GitLab Pages custom domains](../project/pages/custom_domains_ssl_tls_certification/index.md) instructions.