Add a second TXT entry for domain verification

What does this MR do?

This adds a piece of content to the troubleshooting section with regards to verifying custom domains.

This is supported as far as I can tell from various help sites as well as this article: https://about.gitlab.com/handbook/support/workflows/verify_pages_domain.html

In my case the custom domain had not verified after 16 hours but after I added the new TXT entry it verified almost immediately.