sankey diagram default node positions

[chriddyp]

There are a lot of backlinks to plotly/plotly.js#2520 (see e.g. https://community.plot.ly/t/big-sankey-diagram-100-sources-and-100-targets/24352/3?u=chriddyp)

It would be great to update that issue with a link to the documentation that covers this feature: plotly/plotly.js#3583. Then, it'd be great to close out that issue and open a new issue with the remaining items.

[chriddyp]

cc @nicolaskruchten @Mahdis-z