Documentation: More detail on valid plan parameters and how to type various constructions

The documentation [on allowed data types](https://github.com/DiamondLightSource/blueapi/blob/main/docs/explanations/plans.md#allowed-argument-types) defers a lot of explanation to the pydantic documentation, which doesn't mention about nested models (in that section, it does [elsewhere](https://docs.pydantic.dev/dev/concepts/models/#nested-models)) and does not provide any examples of how that maps to the `params` that are passed to run the plan.

This has led to uncertainty about what is supported. These docs should be improved and more examples given, including a fully worked example of writing a plan with nested parameters.

## Acceptance Criteria
- The docs are sufficient for someone new to writing bluesky plans to be able to know how to support their use case