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/customize/components.md

@@ -64,14 +64,14 @@ In your CSS, you'd have something like the following where the bulk of the styli
 
 For the callouts, that unique styling is just a `border-left-color`. When you combine that base class with one of those modifier classes, you get your complete component family:
 
-{{< callout info >}}
+{{% callout info %}}
 **This is an info callout.** Example text to show it in action.
-{{< /callout >}}
+{{% /callout %}}
 
-{{< callout warning >}}
+{{% callout warning %}}
 **This is a warning callout.** Example text to show it in action.
-{{< /callout >}}
+{{% /callout %}}
 
-{{< callout danger >}}
+{{% callout danger %}}
 **This is a danger callout.** Example text to show it in action.
-{{< /callout >}}
+{{% /callout %}}