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/alerts.md

@@ -22,9 +22,9 @@ Alerts are available for any length of text, as well as an optional close button
 {{< /alerts.inline >}}
 {{< /example >}}
 
-{{< callout info >}}
+{{% callout info %}}
 {{< partial "callout-warning-color-assistive-technologies.md" >}}
-{{< /callout >}}
+{{% /callout %}}
 
 ### Link color
 
@@ -70,9 +70,9 @@ You can see this in action with a live demo:
 </div>
 {{< /example >}}
 
-{{< callout warning >}}
+{{% callout warning %}}
 When an alert is dismissed, the element is completely removed from the page structure. If a keyboard user dismisses the alert using the close button, their focus will suddenly be lost and, depending on the browser, reset to the start of the page/document. For this reason, we recommend including additional JavaScript that listens for the `closed.bs.alert` event and programmatically sets `focus()` to the most appropriate location in the page. If you're planning to move focus to a non-interactive element that normally does not receive focus, make sure to add `tabindex="-1"` to the element.
-{{< /callout >}}
+{{% /callout %}}
 
 ## JavaScript behavior