docs: document boilerplates-ref notice behavior

[kitsuyui]

Summary

• Document when the boilerplates-ref GitHub Actions notice is emitted.
• Clarify that the notice is informational and how to suppress it by pinning boilerplates-ref.

Verification

• actionlint .github/workflows/*.yml
• YAML parse for action.yml and workflow files
• Extracted the composite action shell payload from action.yml and ran bash -n
• typos README.md action.yml .github/workflows/*.yml
• git diff --check