Improve callout shortcode. (#31802)

Remove the `markdownify` call, and instead rely on Hugo's proper syntax; `{{% callout %}}` when we want to the content to be processed as Markdown.

This allows for stuff like:

{{% callout info %}}
##### I'm an info callout!

```css
.foo {
  color: #fff;
}
```

{{< example >}}
<div class="alert alert-warning alert-dismissible fade show" role="alert">
  <strong>Holy guacamole!</strong> You should check in on some of those fields below.
  <button type="button" class="btn-close" data-dismiss="alert" aria-label="Close"></button>
</div>
{{< /example >}}
{{% /callout %}}

site/content/docs/5.0/layout/breakpoints.md

@@ -164,9 +164,9 @@ These mixins take those declared breakpoints, subtract `.02px` from them, and us
 // No media query since the xxl breakpoint has no upper bound on its width
 {{< /highlight >}}
 
-{{< callout warning >}}
+{{% callout warning %}}
 {{< partial "callout-info-mediaqueries-breakpoints.md" >}}
-{{< /callout >}}
+{{% /callout %}}
 
 ### Single breakpoint