[Feature Request] Support alert styles that compatible to `learn.microsoft.com`

**Is your feature request related to a problem? Please describe.**

Currently docfx supports [Alerts](https://dotnet.github.io/docfx/docs/markdown.html?tabs=linux%2Cdotnet#alerts) by using [Bootstrap's alert styles](https://getbootstrap.com/docs/5.3/components/alerts/#examples)

Recently GFM(GitHub Flavored Markdown) supports same alert syntax.
But its styles are similar to the styles of `learn.microsoft.com'. And supports more background color variations.
- [GitHub alert style](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts)
- [`learn.microsoft.com` alert style]( https://learn.microsoft.com/en-us/contribute/content/markdown-reference#alerts-note-tip-important-caution-warning])

**Describe the solution you'd like**
Add support for GitHub or `learn.microsoft.com` compatible alert styles.

**Additional context**
These features might be implemented by markdig as custom extensions. (https://github.com/xoofx/markdig/issues/774)