doc(kic) add Gateway API concepts

[rainest]

Summary

Adds a new Gateway API page under the KIC Concepts section. This covers two alpha limitations where Kong's implementation differs from a hypothetical standard Gateway API implementation:

• Gateways are not provisioned automatically and require configuring Kong to match their listeners.
• A Kong instance can only support a single GatewayClass with a single Gateway.

Update the Gateway API guide with links to the new concepts page.

Update the Gateway API guide to include an example of a user-provided certificate.

Reason

Fix #4054
Fix #4229

Testing

Awaiting preview.

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[rainest]

https://deploy-preview-4230--kongdocs.netlify.app/kubernetes-ingress-controller/latest/guides/using-gateway-api/#add-a-gatewayclass-and-gateway
https://deploy-preview-4230--kongdocs.netlify.app/kubernetes-ingress-controller/latest/guides/using-gateway-api/#alpha-limitations

https://deploy-preview-4230--kongdocs.netlify.app/kubernetes-ingress-controller/latest/concepts/gateway-api/

[Guaris]

Is this ready to be reviewed?

[rainest]

@Guaris should be. We may get a few more comments from the rest of the Kubernetes team on tech stuff, but those hopefully don't result in huge changes. Not urgent, so if you want to wait a few days for any of those to trickle in before giving it a final pass that's fine.

[randmonkey]

LGTM here, so removed my change requests. But did not directly approve, to wait for others' reviews
ignore this -- dupelicated.

[randmonkey]

LGTM here, so removed my change requests. But did not directly approve, to wait for others' reviews.

[pmalek]

blocking to get the whitespaces resolved