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/components/button-group.md

@@ -18,13 +18,13 @@ Wrap a series of buttons with `.btn` in `.btn-group`.
 </div>
 {{< /example >}}
 
-{{< callout warning >}}
+{{% callout warning %}}
 ##### Ensure correct `role` and provide a label
 
 In order for assistive technologies (such as screen readers) to convey that a series of buttons is grouped, an appropriate `role` attribute needs to be provided. For button groups, this would be `role="group"`, while toolbars should have a `role="toolbar"`.
 
 In addition, groups and toolbars should be given an explicit label, as most assistive technologies will otherwise not announce them, despite the presence of the correct role attribute. In the examples provided here, we use `aria-label`, but alternatives such as `aria-labelledby` can also be used.
-{{< /callout >}}
+{{% /callout %}}
 
 These classes can also be added to groups of links, as an alternative to the [`.nav` navigation components]({{< docsref "/components/navs" >}}).