Add Gateway API kn operator CLI docs

[kahirokunn]

Summary

• Add Gateway API tab to the Knative Operator CLI plugin installation docs (knative-with-operator-cli.md)
• Documents the kn operator enable ingress --gateway-api command

This is the CLI counterpart of the Operator-side Gateway API support added in knative/operator#2251 (merged 2026-03-19).

Note: This PR depends on the Operator docs PR (#6591) being merged first, and requires the kn-plugin-operator Gateway API support to be released.

Related:

• add net-gateway-api ingress support operator#2251 — Operator-side implementation (merged)
• https://github.com/knative-extensions/kn-plugin-operator — CLI plugin

Test plan

• Verify the Gateway API tab renders correctly in knative-with-operator-cli.md
• Verify the link to configuring-serving-cr.md#configure-the-gateway-api-ingress works

/cc @houshengbo

[knative-prow]

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