Move CUDNN_ROOT_DIR and CHAINERX_CUDNN_USE_CUPY docs to "CUDA support" section
Move CUDNN_ROOT_DIR and CHAINERX_CUDNN_USE_CUPY docs to "CUDA support" section

hvy/chainer,chainer/chainer,okuta/chainer,niboshi/chainer,okuta/chainer,hvy/chainer,niboshi/chainer,hvy/chainer,wkentaro/chainer,okuta/chainer,chainer/chainer,chainer/chainer,wkentaro/chainer,pfnet/chainer,niboshi/chainer,wkentaro/chainer,hvy/chainer,chainer/chainer,okuta/chainer,wkentaro/chainer,niboshi/chainer