abstract data model (Merkle DAG) documentation

We should have a (implementation-independent) description of the SWH data model, to serve a few use cases/publics:

researchers/archivists interested in understanding out how we archive software
developers interested in implementing the same data model in languages other than Python/SQL

This piece of doc can be shipped as part of swh.model and is, in fact, already references by the architecture description (see target link: https://docs.softwareheritage.org/devel/swh-model/data-model.html#data-model); it was just missing a dedicated task before now.

Migrated from T787 (view on Phabricator)