High level doc

Apart from re-organising the documentation somewhat, I've added in some high level domain concept documentation.

I get the feeling that there is a bit of "fear" of working on AATAMS among the development team - but honestly - it's quite a simple application in terms of the domain model and associated business logic.

The tricky part with AATAMS is around the size and performance of the app and backing database, and that's something that *really* needs to be addressed (see https://github.com/aodn/aatams/issues/72) since no significant architectural or design changes have been made since the number of records has grown by several orders of magnitude.