Add demo for Sankey diagram

[hcpchris]

Hi @hcpchris,

Could I suggest to maybe add a demo for a Sankey diagram in the demo repository ?
https://github.com/highcharts-for-python/highcharts-for-python-demos

Because to try to understand how it works, I took a look at the JS examples:
https://www.highcharts.com/samples/highcharts/demo/sankey-diagram?jsfiddle
And as you can see, in JS the tooltip is at the same level as series, so it's a bit confusing.
Especially since pointFormat works perfectly at this level.

Also the workflow in JS seems to be heavily oriented about using a pattern with keys:

keys: ['from', 'to', 'weight'],
data: [
    ['Solar', 'Electricity & Heat', 0.48],
    ...,
],

which is not implemented yet in Python no ? or I could not get it to work.

Thanks a lot for your help,

Originally posted by @JulienBacquart in highcharts-for-python/highcharts-core#148 (comment)