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/layouts/shortcodes/callout.html

@@ -6,5 +6,5 @@
 {{- $css_class := .Get 0 | default "info" -}}
 
 <div class="bd-callout bd-callout-{{ $css_class }}">
-{{ .Inner | markdownify }}
+{{ .Inner }}
 </div>