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/spinners.md

@@ -36,9 +36,9 @@ The border spinner uses `currentColor` for its `border-color`, meaning you can c
 {{< /spinner.inline >}}
 {{< /example >}}
 
-{{< callout info >}}
+{{% callout info %}}
 **Why not use `border-color` utilities?** Each border spinner specifies a `transparent` border for at least one side, so `.border-{color}` utilities would override that.
-{{< /callout >}}
+{{% /callout %}}
 
 ## Growing spinner