First cut at custom hover docs

[rreusser]

A first cut at dash docs for custom hover content. I've created a codepen here, which I can imagine being used for the real thing in order to provide a basic template for CSS which would apply in the majority of cases: https://codepen.io/rsreusser/pen/RwoBXpK (though I'm not a PRO user on CodePen so can't host the CSS assets there.)

More details to follow.