ci: verify example YAML reference headers

[JoeyC-Dev]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

• Detect changed example YAML and Markdown files in pull requests.
• Verify #$ reference headers in example YAML files.
• Report missing, stale, duplicated, and unreferenced example YAML references.
• Write the verification report to the GitHub Actions job summary.

Note:

• The check is skipped when neither example YAML files nor reference Markdown files are changed.
• If only files under examples/ are changed, only the changed example YAML files are checked.
• If any Markdown file is changed, the full example YAML check runs.
• Unreferenced example YAML files are reported but do not fail the check.

Demo: (link may not be permanent)
https://github.com/JoeyC-Dev/gateway-api-example-reference-audit-workflow-demo/pulls

Meantime, job that is skipped will report its status as "Success". So the workflow will not block merging if it is being skipped. (src)

Which issue(s) this PR fixes:

N/A

Does this PR introduce a user-facing change?:

NONE

Declaration:
AI was used to help rephrase warning/error messages, review summary (in short, any text/format related part) and to draft the regular expressions.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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

Hi @JoeyC-Dev. Thanks for your PR.

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