Improve support for GFM alerts

[chalin]

Tasks

• Adjust the mapping from alert type to Bootstrap alert classes. E.g., important shouldn't map to danger. Adjust GFM alert-type to alert-style mapping and more #8863
• Ensure GFM alerts work well for Portuguese - Fix i18n GFM alert title translations for Portuguese #8864
• Improve GFM alert support with md in title, and more #8865
  • Cleanup / drop placeholder for zh-CN, let locale contributors submit the necessary updates
  • chore: cleanup en i18n multiline formatted entries #8867
• Style-guide: correct md alert syntax description and add guidance #8869

Related

• Broken styling for GitHub Markdown alerts in Specification pages #8658
• Support the use of GitHub markdown alerts for the specs #5557