Document how people can make their Policy types recognizable by gwctl

[gauravkghildiyal]

At the moment, we are lacking documentation on how people can integrate the custom Policies that they are defining with gwctl.

The use of the gateway.networking.k8s.io/policy label is (ref) has mentions in a few GEPs but is lacking visibility beyond those places.

Action items that we can begin with:

• Ideally, we want to include this information in the gwctl repo README
• Also, it'd be nice if we can find a place in the Gateway API website (https://gateway-api.sigs.k8s.io/) -- maybe within the guide since it also talks about the BackendTLSPolicy