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/forms/overview.md

@@ -55,11 +55,11 @@ Here's a quick example to demonstrate Bootstrap's form styles. Keep reading for
 
 Block-level or inline-level form text can be created using `.form-text`.
 
-{{< callout warning >}}
+{{% callout warning %}}
 ##### Associating form text with form controls
 
 Form text should be explicitly associated with the form control it relates to using the `aria-describedby` attribute. This will ensure that assistive technologies—such as screen readers—will announce this form text when the user focuses or enters the control.
-{{< /callout >}}
+{{% /callout %}}
 
 Form text below inputs can be styled with `.form-text`. If a block-level element will be used, a top margin is added for easy spacing from the inputs above.