Document user-implementable interfaces and their expected lifecycle

### Description

There are many public interfaces in Altinn.App.Core, many of them are meant to be implemented by app developers. It would be nice if these were documented centrally somewhere. We should also document when they are invoked (generally and with respect to other similar interfaces).

### Additional Information

Common questions/issues
* Users have some business-level need, can be hard to know which interface is appropriate
* What are the expected operations that the developer can invoke on the inputs of the interface method (i.e. can/should you mutate data on a validator invocation)
* Invocation and lifecycle of the interfaces

Could we make a sequence diagram for common operations such as `process/next`?

Docs have theme-based pages (validation, data processing), but no central registry. Maybe this is a good case for a user guide or something?

### Tasks

* Make a list of interfaces that we should document

### Acceptance Criterias

TBD - need refinement