docs: merge version skew protection into the atomic deployments page

Version skew protection and atomic deployments now share one docs page. `/deployment/version-skew-protection` redirects to `/deployment/atomic-deployment`, which explains version skew protection first, states the release it is available from (v4.5.12), and adds a sequence diagram of a run waiting for its matching deployment. A closing section describes the legacy Vercel atomic deployments, marks them as deprecated and not advised, and lists the migration steps.

Mono-RevId: ac02e72b5bb0f0f21a233bf58050db017e91bfc0