Add IPv6 to GitLab Pages custom domain docs

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

What does this MR do and why?

Add IPv6 to GitLab Pages custom domain docs. It is supported, it should be documented. The issue was already linked, but it has been resolved for a while.

See the commit messages for details.

References

• https://docs.gitlab.com/user/project/pages/custom_domains_ssl_tls_certification/
• #214718 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.