Add Operator disconnected OpenShift topic to global nav

What does this MR do and why?

Adds a "Disconnected OpenShift install" entry to the Operator section of the global navigation, after "Ingress in OpenShift". The Operator MR at gitlab-org/cloud-native/gitlab-operator!1784 (merged) adds the page it points to, operator/openshift_disconnected/. The page covers installing the GitLab Operator and a GitLab instance on an OpenShift cluster without access to public registries.

Please merge this only after the Operator MR merges. Until then the page is not on the docs site, so the entry would point to nothing and the link check in scripts/check-navigation.sh fails in this MR's pipeline.

Only the en-us file changes. The ja-jp navigation has its own Operator section, and AGENTS.md leaves the translated navigation to the localization team.

An AI coding assistant wrote this change. It checked each claim against its source: this project's AGENTS.md, the navigation files and the Operator branch. It ran the duplicate, index.html, absolute path and anchor checks from scripts/check-navigation.sh and parsed data/en-us/navigation.yaml as YAML, on a local checkout.

How to set up and validate locally

1. Check out this branch.
2. Run make clone-docs-projects, then make view.
3. Check that the left navigation under Operator lists "Disconnected OpenShift install" after "Ingress in OpenShift". The link resolves once the Operator MR is merged.

Merge request acceptance checklist

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