update docs for ingress2gateway

[justindavies]

Description

Point users to ingress2gateway in the Ingress docs.

Preview Links

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

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[Copilot]

Pull request overview

This PR updates the {{ site.operator_product_name }} how-to for routing traffic with Kubernetes Ingress resources to point readers to the ingress2gateway migration path for moving from Ingress to the Kubernetes Gateway API.

Changes:

• Adds a short note linking to the Ingress-to-Gateway API migration guide (ingress2gateway) from the Operator Ingress how-to.