Update docs for gitlab-pages http_proxy setting

What does this MR do?

omnibus-gitlab!4296 (merged) has modified (with backwards compatibility) the preferred way of setting the http_proxy setting for gitlab-pages. This MR updates the documentation to match

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

Availability and Testing

N/A

Security

N/A