Document design approaches of individual modules and extension modes

[odrotbohm]

Various modules in Salespoint use different designs in terms of which DDD building blocks they implement. Some solely work with an aggregate and repository (e.g. inventory, catalog), others expose a service implementation in front of those (e.g. order, user accounts). Those design decisions result in different ways to extend the module.

We should give a high-level overview about those approaches and document what steps to take if need arises to extend those.