Expose the versioned pages site URL during the ci build

As part of the introduction of the GitLab Pages Multiple Versions Support we need to expose the versioned site URL during the ci build to make it possible to use it when creating links in the site. (The work done in the PoC can be used as reference)

This should only be available on Enterprise Edition when the feature flag pages_multiple_versions is enabled and the ProjectSetting#pages_multiple_versions_enabled is enabled.

Proposed solutions

Update the existing CI_PAGES_URL to show the full url, with the path prefix