Draft Thrust developer documentation

CUB has an [extensive documentation](https://nvidia.github.io/cccl/cub/developer_overview.html) for developers which is very helpful for non-seasoned CUB developers, new team members and contributors. We should have a similar overview for Thrust and clarify:

- [x] systems (aka. backends): what belongs to a system, where they live in the directory tree
- [x] how Thrust dispatches a public API call via ADL to a specific backend implementation via execution policies and how the generic system is involved,
- [ ] how Thrust calls CUB and dispatches offset types
- [x] how Thrust iterators differ from standard C++ iterators: what is an iterator system and the iterator traversal. and how this is encoded in the iterator category