Add integration level persisted variables documentation

What does this MR do and why?

This merge request adds the integration variables in the persisted variables list. We invested a decent amount of time investigating why HARBOR_* variables cannot be passed to trigger jobs to find in the end that they exist in the persisted variables declaration https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/ci/build.rb?ref_type=heads#L510-529

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