Add documentation to the model

[led02]

The classes are not well documented as the user tests show.

Documentation should be added to ld_container that describes how the intended use is and what assumptions are made.

Also, after having decided on #401, the parent, index and key properties should be documented to avoid confusion.

[led02]

I think the data model should be explained, but not in the class documentation. We should put it into an ADR but maybe also add some helpful information in the docs (e.g., what expanded JSON-LD is, information on how to obtain it and how it is processed internally... expamles).

Update: A draft is already available https://github.com/softwarepub/hermes/blob/feature/153-refactor-datamodel/docs/source/dev/data_model.md

[sdruskat]

Additionally, the tutorial for implementing plugins (and potentially other parts of the documentation) should be carefully checked and updated for everything that has changed in the data model 😬, e.g., for #363.

Could be its own issue really.