Use Contextual Component pattern to yield multiple blocks in Hero

Right now the `docs-hero` component assumes that the only use-case for using it in block form is to pass in a custom logo. However, users may want to render additional content in the Hero template, such as a set of badges. It would be great to allow multiple yields similar to the following pattern:

```
{{#docs-hero as |hero|}}
{{#hero.top}}
{{/hero.top}}

{{#hero.bottom}}
{{/hero.bottom}}
{{/docs-hero}}
```

An example of how a developer might want to use this: