Update docs to reflect deprecated SOAP API as well as the most current data model

[Ortimis]

The docs are inconsistent when it comes to the SOAP API. If its deprecated, it should be marked as such everywhere. Not sure if it is though?

Also, while we are at it, instead of admitting that the table names are confusing, we could attempt at documenting the whole data model better and make it less confusing. The screen shot of the MaStR data model is only partially helpful, as it does not contain the naming conventions of open-mastr. For instance, it does not show where the table basic_units fits in the model, and its not clear what table the box electricity producing units refers to.
Im only partially qualified to do this (I do find the table names confusing), so with some clarificatory support (@FlorianK13 @nesnoj ?) I'd be happy to submit a PR.

[FlorianK13]

Regarding the deprecation of the API, this is still ongoing in #635

[FlorianK13]

The data model is also worked on in #684 - there we will probably give the option to use the default data model from BNetzA. Other tables, like basic_units are helper tables to make the API download work. They will be removed with the API deprecation as well.
However it could still be relevant after both #635 and #684 are merged to take a second look at the documentation of the data model...