Add docs for Gitlab Pages Redirects

What does this MR do?

Add docs for Gitlab Pages Redirects which was introduced in gitlab-pages!336 (merged) (part of gitlab-pages#24 (closed))

Does this MR meet the acceptance criteria?

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the [Test Planning Process](https://about.gitlab.com/handbook/engineering/quality/test-engineering).
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done"🍉"

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Part of gitlab-pages#24 (closed)