chore: add hack/verify-markdown-links.sh script

[achrefothmani]

What type of PR is this?
/kind feature
/area conformance

What this PR does / why we need it:
This pull request introduces a new Bash script designed to automate the process of checking for broken external links in markdown (.md) files within all directories recursively.

Which issue(s) this PR fixes:

Fixes #3258

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: achrefothmani
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[k8s-ci-robot]

Welcome @achrefothmani!

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[k8s-ci-robot]

Hi @achrefothmani. 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.

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[aroradaman]

/ok-to-test

[achrefothmani]

/test pull-gateway-api-verify

[achrefothmani]

/retest

[achrefothmani]

/retest

[achrefothmani]

/cc @mlavacca

[achrefothmani]

/retest-required

[k8s-ci-robot]

@achrefothmani: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gateway-api-verify	8f381a6	link	true	/test pull-gateway-api-verify

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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. I understand the commands that are listed here.