Towards a libcudacxx developer guide

A lot of the stuff that is in here is redundant with what is in the top-level docs here: https://github.com/NVIDIA/cccl/blob/main/CONTRIBUTING.md#building-and-testing

I'd prefer to consolidate towards unified docs like these.

I do recognize the need and value for having specific instructions for how to use `lit`.

I'd propose deleting this entire file and instead starting a "libcudacxx Developer Guide" document similar to the CUB one's here: https://github.com/NVIDIA/cccl/blob/main/CONTRIBUTING.md#cub

That should include a section on using `lit` directly to invoke tests.

_Originally posted by @jrhemstad in https://github.com/NVIDIA/cccl/pull/2881#discussion_r1849010063_