[DOC] Draft cuda:: namespace unification guide

As a user of CCCL, I would like to have one place to refer to that tells me everything I need to know about migrating my use of `thrust::` and `cub::` to it's replacement in `cuda::`.

This should be similar to the [CCCL 3.0 Migration guide ](https://nvidia.github.io/cccl/cccl/3.0_migration_guide.html).

I would like to see this be exhaustive for each symbol and it's replacement.

If possible, codifying this as script/clang-tidy check would be ideal.