Add documenation about how to active debug logging for gitlab-pages

What does this MR do?

This add documentation about how to active verbose logging of the GitLab Pages daemon as requested in omnibus-gitlab!2534 (merged).

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Requests in omnibus-gitlab!2534 (merged)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

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