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:

cuda-python/cuda_python/docs/source/index.rst

Lines 33 to 34 in a855762

	conduct.md
	contribute.md

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.