Update cuda-python/README.md to describe the metapackage

[vzhurba01]

Our root level cuda-python/README.md is outdated after the metapackage layout change.

We need to update it to describe the project and reference the different subpackages within cuda-python and their personal README's:

1. cuda-python/cuda_bindings/README.md
2. cuda-python/cuda_core/README.md

[leofang]

@vzhurba01 I think this is done in #252? Any other work needed?