[FR] Support Github callouts in HTML preview for `github_document()`

[etiennebacher]

Hello, Github introduced the callout syntax about 1.5 years ago. I would like to use these callouts in the README of some packages, but they are currently not well rendered on the pkgdown website. It looks like this is due to R Markdown. Currently the output is this:

---
output: github_document
---

> [!NOTE]
> Highlights information that users should take into account, even when skimming.

> [!IMPORTANT]
> Crucial information necessary for users to succeed.

> [!WARNING]
> Critical content demanding immediate user attention due to potential risks.

while I would expect this:

Is this something that R Markdown could support?

---

Reprex of this issue with pkgdown: https://github.com/etiennebacher/pkgdown.callout