Support domain-level redirects

Summary

GitLab Pages currently supports simple redirect rules.

In !458 (merged) we introduced support for splats and placeholders. We should look at supporting domain-level redirects.

See https://docs.netlify.com/routing/redirects/redirect-options/#domain-level-redirects

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Proposal:

• Pages feature: !936 (merged)
• Unit tests: !936 (merged)
• Acceptance tests: !936 (merged)
• Documentation: gitlab!138515 (merged)
• Feature Flag issue: #1087 (closed)

Recording: