Callout rendering in `gfm`

[cscheid]

This should be a matter of adding a custom renderer for the Callout node in gfm output.

[cscheid]

See #1495

[cderv]

Just a note that this is still in Beta : github.com/orgs/community/discussions/16925

Let us know what you think and how this helps you provide better documentation. Please note that this is a beta feature that might be subject to change.

With a few updates since May 2022.

I can't find any other mention in Github doc, spec or blog post.

[cscheid]

Good to know. With that said, it's really very easy for us to do, so it wouldn't be hard to change our rendering if they change their syntax.