Add documentation for takes part in ETS

[mabijkerk]

Description
There is a node attribute takes_part_in_ets. Documentation should be added that describes when a node has this attribute and what it does.

Node should have this attribute when they can be assumed to be part of the EU ETS system. This depends both on the sector and on the typical capacity of the node, see this source.

Node with this attribute set to 1 will have CO2 emission costs assigned to them according to the CO2 price. This will be reflected in their marginal costs, when they participate in the electricity market, in the costs data export where the CO2 costs per node are shown, and in the costs charts where the total CO2 emission costs are shown.

To do
Documentation should be added in two places. First in the Costs methods section, explaining how the CO2 emission costs are calculated. The current documentation on this topic there is very limited. Second it should be added to the For contributors page, explaining what the attribute does (see also #158).