docs: add step for creating the GatewayClass

[zhaohuabing]

Summary

Updates the Kubernetes ingress docs to add a dedicated step for creating the eg GatewayClass that the OpenShell chart references. Previously the docs implied Envoy Gateway registered the GatewayClass automatically, but EG helm chart installs only the CRDs and controller, so the GatewayClass must be created explicitly. Also bumps the Envoy Gateway Helm chart version from v1.7.2 to v1.8.1.

Related Issue

N/A — small docs fix.

Changes

• Reworded the "Install Envoy Gateway" section to clarify that the Helm install provides the Gateway API CRDs and controller (not the GatewayClass)
• Bumped the Envoy Gateway Helm chart version from v1.7.2 to v1.8.1
• Added a new "Create the GatewayClass" section with a kubectl apply snippet that creates the eg GatewayClass using the gateway.envoyproxy.io/gatewayclass-controller controller

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[zhaohuabing]

I have read the DCO document and I hereby sign the DCO.

[TaylorMutch]

/ok to test 079963d

[TaylorMutch]

Thank you for the fix!

[TaylorMutch]

Branch check failing is unrelated to your PR

[zhaohuabing]

Hi @TaylorMutch thanks for the quick response and review on this PR. I really appreciate it.