Geo: Replicate new GitLab Pages deploys on secondaries

Edit: If we make this architecture decision gitlab-pages#195 (closed), this issue becomes irrelevant.

We are planning to first resolve https://gitlab.com/gitlab-org/gitlab-ce/issues/45888.

With that in place Geo should automatically replicate the artifact.

This issue is meant to track the next step: Perform the Pages deploy on the secondary.

Acceptance criteria

• A Geo developer can enable the :geo_pages feature, and view newly deployed Pages sites at both the primary and the secondary
• An admin can set up GitLab with 2 Geo nodes on GCP, enable the feature, and view newly deployed Pages sites at both the primary and secondary
• A developer can find documentation for how to get this working