Menu of toctree

## Spec

Source:

```rst
.. toctree::

   about
```

Output (example):

```html
<aside class="menu">
  <p class="menu-label">General</p>
  <ul class="menu-list">
    <li><a>Dashboard</a></li>
    <li><a>Customers</a></li>
  </ul>
</aside>
```

## Refs

- https://bulma.io/documentation/components/menu/