docs: JSONPath usage in EnvoyPatchPolicy

[arkodg]

Description:

Describe the issue.

Support for JSONPath was recently added to EnvoyPatchPolicy to be able to better select elements in a list More in https://github.com/envoyproxy/gateway/pull/3757

Lets add an example in https://gateway.envoyproxy.io/docs/tasks/extensibility/envoy-patch-policy/ highlighting this use case

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Any extra documentation required to understand the issue.

[arkodg]

cc @denniskniep

[denniskniep]

@arkodg you can assign me