[DOC]: Update top-level landing pages

### 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

The top level landing pages for [Thrust docs](https://nvidia.github.io/cccl/thrust/) or [CUB docs](https://nvidia.github.io/cccl/cub/) feel like totally distinct libraries and not part of a unified whole in CCCL. That's because we never really updated them after merging the repos together.

While a more complete unification of overall docs is necessary, the scope of this issue is just to update the following to better reflect that all of these things are just a part of a larger whole and contextualize what a given component offers (why you should care) and when to use it.

- [ ] Updated https://github.com/NVIDIA/cccl/blob/main/cub/README.md
- [ ] Updated https://github.com/NVIDIA/cccl/blob/main/README.md
- [ ] Updated https://github.com/NVIDIA/cccl/blob/main/thrust/README.md
- [ ] Updated https://nvidia.github.io/cccl/libcudacxx/

### 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_