Make GitLab Pages on separate server docs use new Pages role

What does this MR do?

Updates "Running GitLab Pages on a separate server" documentation to use the new pages_role introduced in omnibus-gitlab!4740 (merged)

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content