Add documentation to describe the topology

[mathbagu]

• Do you want to request a feature or report a bug?
  Feature

• What is the current behavior?
  There is no documentation to explain the topology description supported in PowSyBl. We need a functional description, somewhere in the grid model description, and to rework the existing tutorial to follow the new format.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

• What is the expected behavior?

• What is the motivation / use case for changing the behavior?
  Answer to the question of Zack, asked on spectrum: https://spectrum.chat/powsybl/questions/is-there-any-beginners-guides-on-how-to-use-powsybl~e254b24e-1bb7-4183-9baa-736fe7ccb6f4

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)