A two-pixel rule under the header.

## Usage

```vue
<DuxtProgress />
```

## API

No props.

## Notes

A rule rather than a number: on a long reference page the useful question is "am
I near the end", and a percentage answers it worse than a line does. It is
`aria-hidden`, because it says nothing a screen reader cannot already ask the
document.

Measured against the document rather than a scroll container, so it stays correct
on the docs layout and the landing page alike.