docs: note ingress-nginx retirement; stop pinning ancient controller; steer to Gateway API

[aryasoni98]

Problem

Install docs still push Ingress NGINX (including a 2020 controller pin) after the project's retirement. k8gb has supported Gateway API since v0.17, but docs do not prefer it for new deployments.

Evidence

• docs/deploy_route53.md:21 pins controller-v0.40.2 (Oct 2020); latest was controller-v1.15.1
• Ingress NGINX retired March 2026 — no further releases/CVE patches
• NGINX still default path in deploy_infoblox.md, deploy_route53.md, exposing_dns.md, local.md, README.md
• Gateway API covered in docs/resource_ref.md

References:

• https://www.kubernetes.dev/blog/2025/11/12/ingress-nginx-retirement/
• https://www.kubernetes.io/blog/2026/01/29/ingress-nginx-statement/

Proposed fix

• Add retirement note + migration guidance
• Bump or replace the pinned controller URL where NGINX examples remain
• Point new deployments at Gateway API
• Add Gateway API to README "Ingress Integration Modes" row

Acceptance

New-user path clearly prefers Gateway API; any remaining NGINX examples are explicitly legacy and not ancient pins.

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Suggested label: documentation
Part of the v0.20.0 documentation audit.