TOC: Automatic hierarchy based on directories

E.g., given a project `Aporia` with:

```txt
README.md
To Do.md
Characters/
Characters/Ash.md
Characters/Viola.md
Programs/
Programs/AZURE-STRIPE.md
Programs/OVERFALL.md
```

we should get

> Aporia
>
> - Characters
>   - Ash
>   - Viola
> - Programs
>   - AZURE STRIPE
>   - OVERFALL
> - To Do

Bonus points for expand/collapse.