SSN Components documentation cleanup

[leonardocarreras]

Also thanks for adding the documentation on SSN. I think it's already good. Here are some discussion points for the future:

1. My understanding is that section "Overview" is DPsim-implementation-specific, while section "Concepts" is purely about theoretical concepts. I'd suggest moving DPsim-specific details like using real augmented matrix to "Overview" or just removing. But this is maybe just my understanding.
2. As a follow-up I would also add the list of existing EMT SSN components.

Originally posted by @georgii-tishenin in #517 (review)