Add Gateway to core reference from kinds

[rainest]

What type of PR is this?
/kind documentation
/kind api-change

What this PR does / why we need it:
Adds Gateway to the list of kinds ReferenceGrantFrom must support for core support

https://gateway-api.sigs.k8s.io/v1alpha2/references/spec/#gateway.networking.k8s.io/v1alpha2.SecretObjectReference states:

Note that when a namespace is specified, a ReferenceGrant object is required in the referent namespace to allow that namespace’s owner to accept the reference. See the ReferenceGrant documentation for details.
Support: Core

This indicates cross-namespace SecretObjectReferences are required for core support. This doesn't clarify how you should check their validity, though a Gateway ReferenceGrantFrom entry appears correct, as it's the only independent type that can include a SecretObjectReference (via Listener->GatewayTLSConfig). By extension, it appears you must support Gateways in addition to the currently listed Route types.

Does this PR introduce a user-facing change?:

NONE

Maybe? This seems like it was already implied, but just omitted from docs, and this is more a clarification point to say "yes, you should check for a Gateway from rule, not a GatewayTLSConfig from rule". I'm ambivalent as to whether this should instead be:

Added Gateway to the list of core required kinds in ReferenceGrantFrom

[k8s-ci-robot]

Hi @rainest. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: rainest
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[robscott]

/ok-to-test

[robscott]

I thought this looked familiar... I think this is roughly the same as #1181, and my comment there applies here.

[rainest]

Closed in favor of 1181.