imprv: Add GitHub Markdown alerts

[reiji-h]

Note

マージ先は master ではなく統合ブランチとしている

Summary

• GitHub の Markdown 記法の Alert を追加
  参考：https://github.blog/changelog/2023-12-14-new-markdown-extension-alerts-provide-distinctive-styling-for-significant-content/

Task

• https://redmine.weseek.co.jp/issues/153035
  • https://redmine.weseek.co.jp/issues/153036

ScreenShot

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Latest commit: e177f46

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