Demo: create a realistic application that demonstrate the usage of the lib

[sim51]

For now the demo just display a graph with a few feature. I would like to show, how to :

• do a drag'n'drop
• change the layout algo
• styling the graph
• change / load a new graph

[sim51]

The example module is a pure CRA, with some examples : https://github.com/sim51/react-sigma/tree/main/packages/examples