Update docs to reflect redirects being enabled by default

What does this MR do?

Update docs to reflect redirects being enabled by default

• Redirects feature toggle enabled by default in gitlab-pages!367 (merged)
• Original docs added in !42011 (merged)

Conformity

• Changelog entry
  • "Any docs-only changes should not have a changelog entry."
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content