Review capacity margin feature

[datejada]

Is your feature request related to a problem? Please describe.

Review the capacity margin constraint in the model

• Documentation of the feature (e.g., example, how-to, what is considered, what don't)
• Review how to consider consumption units in the capacity margin constraint

Describe the solution you'd like

• A new section(s) on the docs
• Refactor the constraints such that consumption units are consider in the capacity margin: E.g., if there is an electric heating device connected to an electricity node that has a capacity margin requirement, the electricity consumption of that unit must be taken into account in the constraint.

Describe alternatives you've considered

• Not supporting the consumption units in the constraint and include in the docs that for those cases the minimum capacity margin should be set high enough to cover the consumption of these type of units

Additional context
This issue comes from the review of the parameters in SpineOpt in the Mopo project from @nhniina