Add callout shortcode

[viren-nadkarni]

Background

#452 and #625 made an attempt to make alert boxes consistent with respect to title, colour and styling. However, there's still confusion around its use, and there are instances of colours being mixed with different levels or titles.

Changes

This PR introduces a new callout shortcode to address this.

{{< callout "tip" >}}                                         
If you get so and so error, you can do that.                                                                      
{{< /alert >}}

This shortcode supports three severity levels:

• warning For actions that are destructive/irreversible, have security or other damaging implications

  

• tip For hints and tips

  

• note (default) Used for notices, informative points or everything else.

  

The titles of these alert boxes are determined based on severity levels. If any other argument is passed for level or it is omitted, the default level is used.

I'm hoping these three levels should be sufficient for all severity levels. If required, adding new levels will be trivial.

[github-actions]

🎊 PR Preview has been successfully built and deployed to https://localstack-docs-preview-pr-1260.surge.sh 🎊