docs: add DEV CI regional failover runbook

[roi (roivaz)]

ARO-27246
ARO-27248

What

Add an actionable DEV CI regional failover and failback runbook covering:

• provision-health drain, fallback eligibility, and re-enable criteria
• the e2e-parallel, e2e-parallel-inplace-upgrade, and /test capz-e2e-dev region switches
• prerequisites, decision points, release-config procedure, validation, rollback, ownership, and escalation
• the boundary between automated health signals and required manual operator actions

Link the runbook from the CI overview, monitoring, and operations documentation.

Why

DEV CI already probes westus3, centralus, and canadacentral and alerts on sustained provision failure, but it does not automatically drain or re-enable a region. Operators need one technically validated procedure that turns those signals into a consistent manual decision and updates every affected presubmit together.

Testing

Documentation-only change; no product tests were added.

• Rendered the dev/ci01 partial configuration successfully for westus3, centralus, and canadacentral.
• Validated relative Markdown links in all changed files.
• Ran git diff --check.
• Confirmed the repository has no dedicated documentation check target.
• Cross-checked alert thresholds and retention against ARO-HCP config and the live job names, variables, schedules, workflows, and generated Prow config against current openshift/release origin/main.

Special notes for your reviewer

This PR is intentionally opened as a draft. Reviewers have not been requested yet.

PR Checklist

• PR is scoped to a single task (no mixed concerns)
• Title follows Conventional Commits format
• Summary explains the "Why" behind the change
• Linked to relevant ticket/issue
• Screenshots included (if graph/UI/metrics changes) — not applicable; documentation only
• Self-reviewed the diff
• CI/CD checks are passing (ignore Tide)
• Draft PR used for WIP (if applicable)
• Commit history is clean (rebased/squashed)
• Tricky code blocks are commented — not applicable; documentation only
• Specific reviewers tagged — intentionally deferred while draft
• All comment threads resolved before merge

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

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[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all