docs(subnet-discovery): mention manual subnet annotations

[alliasgher]

Issue

Fixes #3776

Description

The subnet auto-discovery guide currently doesn't mention that subnets can be set explicitly via annotations, so users who want to override discovery (the original reporter's case: limiting an LB to specific subnets) have to dig through both the Ingress and Service annotation pages to find the right option.

This adds a short "Manual override" tip box at the top of the page with direct links to:

• `alb.ingress.kubernetes.io/subnets` (Ingress annotation reference)
• `service.beta.kubernetes.io/aws-load-balancer-subnets` (Service annotation reference)

…and notes that setting either annotation skips auto-discovery entirely.

The other half of the issue (a unified, searchable annotation list) is a larger docs restructuring; this PR only addresses the discoverability problem the reporter explicitly called out.

Checklist

• Added/modified documentation as required (such as the `README.md`, or the `docs` directory)
• Made sure the title of the PR is a good description that can go into the release notes

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