Improvements in the documentation

[JulStraus]

Node description

While the node description is still valid, it is in my opinion slightly more difficult to follow than required. I would suggest to reformulate it to be more precise regarding the individual constraint functions and what they entail as well as what the preferred options are for creating a new Node, depending on the desired features.

Link description

While we allow for subtypes to Link (and even use it in EnergyModelsHeat for DhPipe), we do not have a proper description in the documentation. This makes it significantly more difficult for people to see what is possible with links. My thought process is to base it on the description of Nodes,