Add an example of using popper.js to show a tooltip in a scatterplot

[wlach]

@cnwangjie did up a great demo of how it's already possible to use popper.js in combination with the mouseover event to show a tooltip when elements of a scatterplot are hovered over:

https://jsfiddle.net/wangjie/6cp98pcp/

We really should update the docs to show how this works. We probably need to reorganize the documentation categories in general, but for now putting this under "experimental" (where the other scatterplot stuff is) should be fine.

[cnwangjie]

@wlach Hi, I submit a PR (#894) to add an example just now. It could be used in any chart type with mouseover option. So I create the example with a line chart and a scatterplot, I think it can just show how it works. Could you take a look?