Add documentation for relevant node attributes

[mabijkerk]

Background
The node source analyses contain attributes like free_co2_factor and modellers often wonder whether how to interpret these. It would be useful to maintain documentation that explains the function of attributes like this, as well as stating whether they are deprecated or not.

Goal
Add documentation about node attributes to the "For contributors" page. ETSource only contains a short reference to input and output conversions. Documentation should include an explanation of the attribute and whether it is still used.

Focus should be on generic attributes that are used in most nodes, but whose function does not immediately become clear from the name. For example, initial_investment or output.loss are rather clear, so don't have priority, but free_co2_factor is less clear.

Non-exhaustive list of relevant attributes:

• Free_co2_factor
• Land_use_per_unit
• Takes_part_in_ets
• Energy_balance_group (?)
• Forecasting_error (deprecated)
• Part_load_efficiency_penalty (deprecated)
• Part_load_operating_point (deprecated)
• Hours_maint_nl (deprecated)
• Hours_place_nl (deprecated)
• Hours_prep_nl (deprecated)
• use = energetic/non_energetic/undefined
• waste_outputs
• ...

Notifying @thomas-qah and @redekok