Docs: Update Quickstart Released/Main Version Kgateway Refs

[danehans]

#1771 updates the quickstart guides to use kgateway with the agentgateway data plane. The versioned guide continues to use the following versioned refs:

kubectl apply -f https://raw.githubusercontent.com/kubernetes-sigs/gateway-api-inference-extension/refs/tags/v1.0.2/config/manifests/gateway/agentgateway/gateway.yaml

kubectl apply -f https://raw.githubusercontent.com/kubernetes-sigs/gateway-api-inference-extension/refs/tags/v1.0.2/config/manifests/gateway/agentgateway/httproute.yaml

When the v1.2.0 release is cut, these links should be updated to the following since the config/manifests/gateway/agentgateway directory will no longer exist when #1771 is merged:

kubectl apply -f https://raw.githubusercontent.com/kubernetes-sigs/gateway-api-inference-extension/refs/tags/v1.0.2/config/manifests/gateway/kgateway/gateway.yaml

kubectl apply -f https://raw.githubusercontent.com/kubernetes-sigs/gateway-api-inference-extension/refs/tags/v1.0.2/config/manifests/gateway/kgateway/httproute.yaml