add support for NOTE, WARNING and IMPORTANT github markdown syntax

[9glenda]

Render Github markdown syntax

> [!IMPORTANT]
> Render it

[!IMPORTANT] Render it

[!NOTE] 
Render it

[!NOTE] Render it

[!WARNING] 
Render it

[!WARNING] Render it

Example

Example of how it is done currently.

[domenkozar]

cc @sandydoo maybe we just need to update the github markdown parser?

[sandydoo]

There's probably an extension for it. I'll look into it.

[sandydoo]

We can use marked-alert for the rendering and update the CSS generator to include the alert styling.

[sandydoo]

🎉 Now to figure out how to ship this...

[sandydoo]

Now upstreamed to github-markdown-css.