#2836 Docs: manual approval gates inside nested workflows (swamp-club#2736)

swamp-club#2736 changes how a manual_approval gate inside a nested workflow behaves. When the child workflow suspends at a gate, the parent run suspends too, on the step that runs the child. The child stays an ordinary suspended run: approve, reject, resume and cancel it under its own workflow name and run id, then resume the parent, which reads the child's outcome (a succeeded child's outputs are adopted; a rejected gate fails the parent step as a rejected approval). Resuming the parent first is refused with the commands that settle the child. Under swamp serve, the parent resumes on its own once the child finishes, under the parent workflow's auto-resume policy and only if the approver holds the approve grant on the parent. Cancelling or rejecting the parent leaves the child suspended and prints the command that cancels it (cascading that cancel is swamp-club#2867). Pages to update: content/manual/how-to/gate-a-workflow-with-manual-approval.md (add a nested-gate section with the three commands) and content/manual/reference/workflows.md (nested workflow steps and manual_approval).