Support domain-level redirects
### Summary
GitLab Pages currently supports simple redirect rules.
In https://gitlab.com/gitlab-org/gitlab-pages/-/merge_requests/458 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:
- [x] Pages feature: https://gitlab.com/gitlab-org/gitlab-pages/-/merge_requests/936
- [x] Unit tests: https://gitlab.com/gitlab-org/gitlab-pages/-/merge_requests/936
- [x] Acceptance tests: https://gitlab.com/gitlab-org/gitlab-pages/-/merge_requests/936
- [x] Documentation: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138515
- [x] Feature Flag issue: https://gitlab.com/gitlab-org/gitlab-pages/-/issues/1087
Recording:

issue