Additional verification flows for Pages custom domains
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=20948) </details> <!--IssueSummary end--> The first implementation for the verification of a custom domain in Pages (https://gitlab.com/gitlab-org/gitlab-ce/issues/29497) uses DNS. We can consider additional verification methods for users that cannot set up a TXT entry to validate the domain. A possible solution is a verification code sent to an email address that is linked to the owner of the domain (whois). Another option is to improve dns validation with additional resources, like `CNAME` entries.
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