Documentation improvements

[Regis-Delubac]

In the tutorials, it is often not very clear how a feature works exactly.
I will use the reserve tutorial as a reference (https://spine-tools.github.io/SpineOpt.jl/latest/tutorial/reserves/).

Why do we use a group of nodes? (The note explains that it helps involve variables from its members.) Why don’t we just use the upward reserve node?

What does the function "is reserve node" do? You say it indicates that the node is a reserve node, but how is a reserve node different from a balance node, for example? (The second note partially helps.)

The second note says: "The value is equal to the unit capacity defined for the electricity node. However, the value can be lower if the unit cannot provide reserves with its total capacity."
This shows that the reserve node is indeed weaker than the demand node (which is a good thing), but how can we determine/understand the penalty and constraints related to not providing reserves?

I think it would be very helpful to clarify why each aspect is modeled the way it is in the tutorial so that the approach can be extended to larger models.