Anchor.js for Gitbook

Adds deep links to headlines using AnchorJS v4.1.0.

``js
{
"plugins": ["anchorjs"]
}
`

The deep links to page sections are initially-hidden and pop into view on hover.
Design and behavior can adjusted in book.json:

`json
{
"pluginsConfig": {
"anchorjs": {
"selector": "h2,h3,h4,h5",
"icon": "#",
"placement": "left",
"visible": "always",
"class": "custom-class another-one",
"truncate": 64,
"ariaLabel": "Anchor"
}
}
}
``

See anchor.js docs for details.