DOC: Add a page showing a case study with an existing network

[alihamdan]

For example if we take one of the networks of the Representative French Grids, the scenario could be as follows:

1. Load the network from the file
2. Visualize the network on a map
3. Run an initial load flow and look at the voltage profile
4. Identify interesting MV loads, LV loads, generators, etc. Show how to filter the loads of chosen buses
5. Create summer and winter load points by modifying the network's loads
6. Plot the voltage profile of both load points on the same graph
7. Report some statistics (line losses, transformer losses, voltage violations, current violations, transformer overloads, total voltage drop/rise, etc.)

@mullerlo feel free to change this list