docs: add SubnetDiscoveryByReachability feature gate to values.yaml and feature-gates table

[nghiadaulau]

Issue

When creating an ALB without specifying subnets and without providing the annotation alb.ingress.kubernetes.io/subnets,
the controller still automatically discovers and attaches subnets — even though those subnets are not tagged with kubernetes.io/cluster/${cluster-name}.

This behavior comes from the SubnetDiscoveryByReachability feature gate, but it is not currently listed in values.yaml or the feature-gates documentation table.

Description

This PR improves documentation and visibility of supported feature gates:

• values.yaml:
  Added a commented-out entry under controllerConfig.featureGates for SubnetDiscoveryByReachability.
  This makes it easier for users to discover and configure this option directly in values without having to dig into the docs.

• Feature Gates Table:
  Added a new row for SubnetDiscoveryByReachability that explains how the controller automatically classifies subnets as public or private based on their route table configuration.
  This ensures consistency and parity with the actual supported feature gates.

These changes are purely documentation/UX improvements and do not modify any controller logic.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested (rendered values.yaml and docs locally)
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[k8s-ci-robot]

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[k8s-ci-robot]

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