KIC: Introduce annotations available for gateway API resources

[randmonkey]

Where is the problem?

https://docs.konghq.com/kubernetes-ingress-controller/latest/reference/annotations/

What happened?

There is no introduction on annotations could be used on gateway API resources, such as HTTPRoute in the annotations page. Basically, HTTPRoute supports the same annotations as Ingress, but since some features like method match and header match are available in specs of HTTPRoute, so we disabled the annotations (when expression router enabled). We need to add introductions about the differences.

What did you expect to happen?

There is a section to introduce annotations could be applied on gateway API resources like HTTPRoute.

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