Update Helm Charts to work with Geo Unified URL feature

Summary

Follow-up to spike #3845 (closed).

We'll need to make some adjustments to the Helm Charts to support Geo's Unified URL feature.

The findings from the spike are in this comment: #3845 (comment 1350184587).

The Helm Chart configurations used for the primary and secondary sites are in this comment: #3845 (comment 1351009781).

Acceptance criteria

• Separate Ingress object is deployed that uses the Unified URL
• A separate NGINX Ingress Controller is deployed to make use of forwarded headers as securely as possible.
• Templates are updated to allow for differentiating between a Release's own domain (global.hosts.domain) and the Unified URL to use (something like global.hosts.gitlab.name).
• Documentation is added to explain how to use this feature.