Architecture: Composite management

As pointed out at https://github.com/DiamondLightSource/hyperion/pull/1110#discussion_r1477980856 we have an issue where if we need a new device at the lower level in the plan we end up having to add it in lots of different composites. We should think about if this is the best way to do this. Maybe we should have one giant beamline composite?

## Acceptance Criteria
* We discuss the architecture of composites
* Decisions about the architecture are documented
* Changes are made to the architecture if we think it's necessary