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/layout/breakpoints.md

@@ -164,9 +164,9 @@ These mixins take those declared breakpoints, subtract `.02px` from them, and us
 // No media query since the xxl breakpoint has no upper bound on its width
 {{< /highlight >}}
 
-{{< callout warning >}}
+{{% callout warning %}}
 {{< partial "callout-info-mediaqueries-breakpoints.md" >}}
-{{< /callout >}}
+{{% /callout %}}
 
 ### Single breakpoint
 

site/content/docs/5.0/layout/columns.md

@@ -6,9 +6,9 @@ group: layout
 toc: true
 ---
 
-{{< callout info >}}
+{{% callout info %}}
 **Heads up!** Be sure to [read the Grid page]({{< docsref "/layout/grid" >}}) first before diving into how to modify and customize your grid columns.
-{{< /callout >}}
+{{% /callout %}}
 
 ## How they work
 

site/content/docs/5.0/layout/grid.md

@@ -10,9 +10,9 @@ toc: true
 
 Bootstrap's grid system uses a series of containers, rows, and columns to layout and align content. It's built with [flexbox](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Basic_Concepts_of_Flexbox) and is fully responsive. Below is an example and an in-depth explanation for how the grid system comes together.
 
-{{< callout info >}}
+{{% callout info %}}
 **New to or unfamiliar with flexbox?** [Read this CSS Tricks flexbox guide](https://css-tricks.com/snippets/css/a-guide-to-flexbox/#flexbox-background) for background, terminology, guidelines, and code snippets.
-{{< /callout >}}
+{{% /callout %}}
 
 {{< example class="bd-example-row" >}}
 <div class="container">