Add more demos and interactive tests

[olexandr-konovalov]

I've updated the Dockerfile and added a binder badge in 05d7210. It starts an instance on Binder with GAP 4.11.1 and the latest version of JupyterKernel from the master branch of this repository. This can be used to demonstrate the package, and to test that interactive features, which we are not testing with CI, are not broken by latest commits.

The collection of demos needs to be extended. At the moment it has only one file with some advanced attempt of graph visualisation. We need to have a demo which starts from basics, explains Jupyter for newcomers, and gradually goes to more advanced features. We may also have notebook(s) for manual tests that they are runnable.

[olexandr-konovalov]

Also, the README has just the badge for Binder - could also explain how to use it.