Make CoC and Contributing doc pages per module

[leofang]

Currently the Code of Conduct (CoC) and Contributing pages are placed under the parent CUDA Python homepage: https://github.com/NVIDIA/cuda-python/blob/a85576273c54098af60483a80b043224239bec35/cuda_python/docs/source/index.rst?plain=1#L33-L34 However, technically this is not right, because each module under cuda could (and do!) have separate CoC and contribution policies (and licenses!). We shouldn't give the impression that there's a uniform policy imposed to all modules.