docs(ingress): add note about OpenShift Gateway API and link to OpenShift guide

Problem Statement

The ingress docs (docs/kubernetes/ingress.mdx) only cover Envoy Gateway. On OpenShift, Envoy Gateway cannot be installed because the Ingress Operator manages Gateway API CRDs and rejects third-party installations. Users following the ingress guide on OpenShift hit an error with no guidance on what to do instead.

Proposed Design

Add a <Note> callout at the top of docs/kubernetes/ingress.mdx directing OpenShift users to the OpenShift-specific guide:

<Note>
On OpenShift, the Ingress Operator manages Gateway API CRDs and Envoy Gateway cannot be installed.
Refer to [OpenShift — Gateway Connection](/kubernetes/openshift/gateway-connection) for OpenShift-specific
ingress options including reencrypt Routes and the Istio-based Gateway API controller.
</Note>

Changes required

• docs/kubernetes/ingress.mdx — add the <Note> after the introductory paragraph (before "## Install Envoy Gateway")

Alternatives Considered

1. Add a full OpenShift section to ingress.mdx: Would duplicate content already covered in docs/kubernetes/openshift/gateway-connection.mdx. A cross-reference note is sufficient.

2. Do nothing, rely on the OpenShift section: Users coming from the Kubernetes nav sidebar would land on ingress.mdx first and not know to check the OpenShift section.

• I've reviewed existing issues and the architecture docs
• This is a design proposal, not a "please build this" request

Start with docs/kubernetes/ingress.mdx and compare the existing OpenShift guidance in docs/kubernetes/openshift/gateway-connection.mdx. Add the specified Note after the introductory paragraph and before "## Install Envoy Gateway". Done means OpenShift users are directed to the OpenShift guide without duplicating its content.