[Documentation] Add a section about powsybl configuration

[sylvlecl]

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

Documentation

• What is the expected behavior?

On the documentation website, we should have a guide about configuration to explain what is and where is the powsybl configuration file.

As much as possible, it should rely on the existing powsybl documentation in order to not maintain a double doc, but we must have at least a few pointers for python users.

• What is the motivation / use case for changing the behavior?

It's very hard (impossible?) for a newcomer to know how to configure powsybl components.