New feature: Support GitHub markdown Alert styles

[rydmike]

For some time GitHub has supported Alerts in markdown with this style:

https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts

> [!NOTE]
> Useful information that users should know, even when skimming content.

> [!TIP]
> Helpful advice for doing things better or more easily.

> [!IMPORTANT]
> Key information users need to know to achieve their goal.

> [!WARNING]
> Urgent info that needs immediate user attention to avoid problems.

> [!CAUTION]
> Advises about risks or negative outcomes of certain actions.

Note

Useful information that users should know, even when skimming content.

Tip

Helpful advice for doing things better or more easily.

Important

Key information users need to know to achieve their goal.

Warning

Urgent info that needs immediate user attention to avoid problems.

Caution

Advises about risks or negative outcomes of certain actions.

Please consider adding them as an alternative to the current Callouts

Pub.dev has started to support the GitHub Alerts markdown too. It makes using them convenient when they are supported in both GitHub and pub markdown docs. It would further simplify things if docs.page supported them too.

[Ehesp]

Good call!