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/utilities/visibility.md

@@ -7,9 +7,9 @@ group: utilities
 
 Set the `visibility` of elements with our visibility utilities. These utility classes do not modify the `display` value at all and do not affect layout – `.invisible` elements still take up space in the page.
 
-{{< callout warning >}}
+{{% callout warning %}}
 Elements with the `.invisible` class will be hidden *both* visually and for assistive technology/screen reader users.
-{{< /callout >}}
+{{% /callout %}}
 
 Apply `.visible` or `.invisible` as needed.