docs(rfc): RFC 0020, Cedar as an alternate sandbox policy language

Track the RFC 0020 document itself: rfc/0020-cedar-policy-engine/README.md, proposing that sandbox policies can be authored in Cedar as an alternative to YAML. Maintainers assigned the RFC number on #4411; the policy work overall is tracked by the parent issue #4474, which stays open.

Scope of this issue is the RFC text in review state and its acceptance. Implementation is tracked by #4411 (PR #4565).