Change CNAME of docs.gitlab.com

We're in the process of transferring https://gitlab.com/gitlab-com/gitlab-docs under the gitlab-org group and we need to change the DNS. The new CNAME should be:

docs.gitlab.com CNAME gitlab-org.gitlab.io.

• This should be done in step 1 https://gitlab.com/gitlab-com/gitlab-docs/issues/310#immediately-after-migration