Add documentation about the Energy sector

[mabijkerk]

Our documentation of demand sectors includes six sectors:

The sector Energy is missing from this list, but is one of our demand sectors in the model:

Users often ask questions about this sector, as it is unclear what's included in the demand. Some users expect electricity production to be represented in this sector. Therefore it is important to add an explanation about the Energy sector in the ETM to our documentation.

[mabijkerk]

In the ETM it may be useful to specifically add "own use" to the label of the Energy sector, in order to better clarify that it is not exactly final demand.