Ensure that shortcode calls use % delimiters if they contain markdown

If the content of a [shortcode][] call is markdown, and you need things like **render-link hooks** to trigger, then use `%` as the shortcode delimiter (excerpt from the Hugo docs):

```nocode
{{% mdshortcode %}}Stuff to `process` in the *center*.{{% /mdshortcode %}}
```

Otherwise, you can use `< >`, like this: `{{< myshortcode >}} ... {{< /myshortcode >}}`

- [ ] Alert shortcodes: treat all bodies as markdown #640

[shortcode]: https://gohugo.io/content-management/shortcodes/