Add documentation about distributed data models

[asishallab]

Our documentation has a high level of detail and is written well for several target audiences.

None the less, one of Zendro's key features is missing documentation, i.e. Distributed Data Models (DDMs).

Please add documentation about DDMs covering

• what they mean and are used for,
• how to define them, and how to define adapters,
• the generated API and why limit-offset-based pagination cannot be implemented efficiently.

For the latter see this presentation, slides six and following for material and explanations. Also consider this meeting log (pages five and six) for a more formal proof of the limit-offset-pagination problem.