Custom policy attachment status reporting doc

*Description*:
Envoy Gateway supports custom policy attachments via Extension server (see example [here](https://gateway.envoyproxy.io/latest/tasks/extensibility/extension-server/)). But it is not clear how the status for the policy attachment be updated.

Ask of this issue is to have a companion doc to EG [extensions design](https://gateway.envoyproxy.io/contributions/design/extending-envoy-gateway/) which details to a vendor authoring custom policies on how to leverage EG extension server to do status reporting for their custom policies.

Looking thru [GEP-713](https://gateway-api.sigs.k8s.io/geps/gep-713), the status reporting on the policy attachments seems like can get very complicated with multiple options floated.

Perhaps we can start with "Standard status struct" scenario ([reference](https://gateway-api.sigs.k8s.io/geps/gep-713/#standard-status-struct)) which is what `BackendTLSPolicy` follows.

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