docs: task version skew protection, --external-id, and the atomic deployments deprecation (#4742)

New deployment/version-skew-protection page: the skew problem, the
--external-id primitive and its reuse behaviour, runtime discovery (call
option, configure(), TRIGGER_EXTERNAL_DEPLOYMENT_ID, and the gated
platform/CI/generic commit-SHA variables with the build-time caveat),
the
manual any-platform recipe, waiting/expiry semantics, precedence, and
automatic skew protection on Vercel. Deprecation callouts on the atomic
deployments page and the Vercel integration page; --external-id/--force
added to the CLI deploy reference; redirect from
deployment/vercel-skew-protection so existing webapp links resolve
(TRI-13002).