Update documentation

[jo-mueller]

There will be quite a handful of changes around the corner with 0.9.0 with the side-effect that much of the documentation will probably be outdated and needs to be rewritten. I suggest to set up a Jupyter book on gh pages for this repo so that we can have all the documentation in one place. The sample data now comes with the plugin, aynway.

Here are some things that should be covered:

• Screenshots and how-tos for Plotter Widget
• Screenshots and how-tos for Dimensionality reduction widget
• Screenshots and how-tos for Clustering widget
• Introduction of across-layer functionality
• Categorical vs. continuous features
• Installation
• How to contribute a new algorithm
• API documentation for widgets?
• API documentation for sample data functions?