Move gateway operator demo bundles out of helm/resources

Description

The gateway operator's end-to-end demo bundles currently live under kubernetes/helm/resources/ (apigateway-restapi-operator-demo, apim-apigateway-restapi-operator-demo, apim-gateway-api-operator-demo, gateway-api-operator-demo, gateway-api-httproute-policies-demo, secure-backend-k8s). These are operator usage demos — multi-file bundles with READMEs, backends, numbered apply sequences, RestApis, and policies — not helm-chart resources, so kubernetes/helm/resources/ is a confusing home for them. They should move to a clearer operator-owned location such as kubernetes/gateway-operator/demos/.

This is distinct from kubernetes/gateway-operator/config/samples/, which holds minimal single-CR reference samples (kubebuilder-managed). Those stay put; only the multi-file demo bundles move.

Files that reference these paths and must be updated as part of the move (13 references across 10 files):

• .github/workflows/operator-integration-test.yml — CI consumes these paths, so it must be updated in the same change or CI breaks.
• kubernetes/gateway-operator/README.md
• kubernetes/gateway-operator/docs/CONFIGURATION.md
• kubernetes/gateway-operator/docs/GATEWAY_API_IMPLEMENTATION_NOTES.md
• The demo bundles' own README self-references (apigateway-restapi-operator-demo, apim-apigateway-restapi-operator-demo, apim-gateway-api-operator-demo, gateway-api-operator-demo, gateway-api-httproute-policies-demo).
• kubernetes/helm/resources/apigateway-restapi-operator-demo/llm-mock-openapi-it/Dockerfile

Considerations:

• These are GA product artifacts, so external tutorials or docs may link the current paths — update known references and consider leaving a short pointer note at the old location.
• Do the move plus all reference updates atomically in a single PR so CI stays green.
• Decide whether kubernetes/helm/resources/kafka-deployment (a shared test fixture with no README) moves too or stays.
• This should be a dedicated PR, not folded into the operator versioning-docs work.

Version

No response

Related Issue

Related #3288

Start by reviewing the six demo directories under kubernetes/helm/resources/ and the 13 references listed in the issue, especially .github/workflows/operator-integration-test.yml and the operator documentation. Move only the multi-file demo bundles to kubernetes/gateway-operator/demos/, update README and Dockerfile self-references, and decide whether kafka-deployment stays. Done means all known paths resolve, CI references the new location, and the distinction from config/samples remains clear.