Section Configuration
    Each folder inside book defines a section. Each folder must contain a index.md. The index file
    can be configured like a page.
  
    You can also make folders virtual. If a folder is declared virtual,
    the folder will not create another level. This is helpful if you want
    to structure your hyperbook with folders, but they should not affect the
    navigation.
  
To summarize, here are the properties you can set in the frontmatter:
| Property | Description | 
|---|---|
| ... | Every Property from a page | 
| virtual | Everything will appear on the same level as the parent. Be aware that the index property does not work across folders. | 
| expanded | Defines whether the section is expanded on page load. | 
    If the index.md file only contains a front matter and is otherwise
    empty, it will appear in the navigation as a non-clickable item.