Make `controlling_em` field mandatory in readthedocs

[sebastian-peter]

Recently, the controlling_em field was made mandatory in PSDM. This isn't reflected in readthedocs yet.

(The field can no longer be missing)

Example: https://powersystemdatamodel.readthedocs.io/en/latest/models/input/participant/wec.html

[staudtMarius]

@sebastian-peter Maybe we should make it clear that every field in the documentation needs to be present in the source. But we should also mention that some fields are allowed to be empty.

[sebastian-peter]

@staudtMarius Is operator and operation interval also mandatory now?

[staudtMarius]

@sebastian-peter The fields for em, operator and operating time can be empty. The only thing that changed is that now all fields needs to be set for example in the csv headline.

[danielfeismann]

solved by #1449