Block components contents clobbered into text only

This code: https://github.com/ember-learn/ember-cli-addon-docs/blob/b4bfe91ad58406808b960fc1870d571b16b462a4/lib/utils/compile-markdown.js#L28-L54 prevents the use of block components to yield to Markdown.

This leads to it being impossible to wrap the Markdown in a component to provide a highlight. For example, this does not work:

```
{{#highlight-this for='engineering'}}

#### Note

I don't recommend actually eating this pineapple.

{{/highlight-this}}
```

Instead of clobbering the markup, can we instead try to preserve it as much as possible?