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]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: nghiadaulau
Once this PR has been reviewed and has the lgtm label, please assign m00nf1sh for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[linux-foundation-easycla]

• ✅login: nghiadaulau / name: Huỳnh Lê Nhất Nghĩa / (6c90223)
• ✅login: nghiadaulau / name: Huỳnh Lê Nhất Nghĩa / (6c90223, 6031dc4)

The committers listed above are authorized under a signed CLA.

[k8s-ci-robot]

Welcome @nghiadaulau!

It looks like this is your first PR to kubernetes-sigs/aws-load-balancer-controller 🎉. Please refer to our pull request process documentation to help your PR have a smooth ride to approval.

You will be prompted by a bot to use commands during the review process. Do not be afraid to follow the prompts! It is okay to experiment. Here is the bot commands documentation.

You can also check if kubernetes-sigs/aws-load-balancer-controller has its own contribution guidelines.

You may want to refer to our testing guide if you run into trouble with your tests not passing.

If you are having difficulty getting your pull request seen, please follow the recommended escalation practices. Also, for tips and tricks in the contribution process you may want to read the Kubernetes contributor cheat sheet. We want to make sure your contribution gets all the attention it needs!

Thank you, and welcome to Kubernetes. 😃

[k8s-ci-robot]

Hi @nghiadaulau. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.