[DOC]: Thrust / Cub Codebase Documentation

### Is this a duplicate?

- [X] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this bug and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Is this for new documentation, or an update to existing docs?

New

### Describe the incorrect/future/missing documentation

Trying to understand the architecture of Thrust and Cub. It would be helpful for documentation on the structure of the codebase, specifically, how the high level abstractions provided by Thrust and Cub are ultimately transformed into primitive cuda memory allocations / kernel launches.

Something akin to the cutlass documentation -- [here](https://github.com/NVIDIA/cutlass/blob/main/media/docs/code_organization.md) and, more generally, [here](https://github.com/NVIDIA/cutlass/blob/main/media/docs/) -- would be ideal.

### If this is a correction, please provide a link to the incorrect documentation. If this is a new documentation request, please link to where you have looked.

_No response_