GFM Alerts / Notes

[Calum Knott]

GFM now allows for markup such as :

Important

This is an important note

Warning

This is a warning

https://github.com/orgs/community/discussions/16925

It is not the same format used by the containers plugin : https://mdit-plugins.github.io/container.html

It will not be supported by markdown-it native : markdown-it/markdown-it#974