Add the Markdown extension Alerts

[mardukbp]

Alerts are very useful for writing tutorials and documentation. GitHub supports the alert types Note, Tip, Important, Warning, Caution. Here's an example:

> [!TIP]
> Use Livebook to get started with Elixir!

Tip

Use Livebook to get started with Elixir!

It would be great if Livebook had this Markdown extension.

[josevalim]

We had discussions about this on ExDoc but I am strongly against GitHub's implementation of alerts for the reasons outlined here. In a nutshell, it introduces English specific words to Markdown, which has none, and completely ignores the case that Markdown is meant to be read as plain text.

If it goes all the way up to CommonMark, then we will support it, but I see no reason to opt-in to a syntax which, according to my interpretation, goes against Markdown goals. :)

[mardukbp]

Excellent points @josevalim! I will just use emojis, which have become part of plain text these days 😁