Update NGF documentation on how to use NginxProxy

[kate-osborn]

Proposed changes

Problem: The documentation for NGF's NginxProxy API needs to be updated now that NGF supports attaching NginxProxy resources to Gateways.

Solution: Updates the Gateway API Compatibility doc to list Gateway Infrastructure field as partially supported. Also, updates the data plane configuration how-to to describe how to attach NginxProxy resources to Gateways and GatewayClasses.

Testing: Checked the rendered documentation.

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