Atlantis production readiness and rollout

Following the Atlantis PoC and demo (#24174 (closed)), we need to make production ready and roll it out.

• Resolve outstanding issues:
  • Locking other MRs from applying while a MR is being applied and not yet merged (ideally keep Atlantis locking enabled but have non-locking draft plans)
  • Daily state drift check (https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24833)
  • Daily state refresh
  • Conftest
  • Monitoring
• Documentation
• Production readiness review
• Plan the rollout
• Rollout and announcement