Document the CEL vocabulary supported in the reference implementation

https://github.com/kubernetes-sigs/kube-agentic-networking/pull/264 proposes removing CEL vocabulary standardization. After the proposal is merged, we should document the CEL vocabulary supported in the reference implementation. 

https://github.com/kubernetes-sigs/kube-agentic-networking/pull/241 adds the CEL support to the reference implementation.