Add support for GitHub-flavored Markdown alerts

[chalin]

References:

• https://gohugo.io/render-hooks/blockquotes/#alerts
• https://docs.github.com/en/contributing/style-guide-and-content-model/style-guide#alerts
• https://github.blog/changelog/2023-12-14-new-markdown-extension-alerts-provide-distinctive-styling-for-significant-content/

Consider driving them from the same map as BS uses to generate its alerts. This way, if a project extends that map, we will take it into account.

[chalin]

/cc @svrnm