New example for location value when using complex Labels

[djsly]

While trying to find how to target a complex/namespaced label with the Mutation CRD's location field, I stumbled on https://github.com/open-policy-agent/gatekeeper/issues/2877 for the solution, it might be good to have such example in the official Mutation CRD doc. https://open-policy-agent.github.io/gatekeeper/website/docs/mutation/