Add `chainerx.ndarray` to the `ndarray` doc
Add `chainerx.ndarray` to the `ndarray` doc

hvy/chainer,hvy/chainer,niboshi/chainer,pfnet/chainer,jnishi/chainer,chainer/chainer,keisuke-umezawa/chainer,wkentaro/chainer,tkerola/chainer,niboshi/chainer,niboshi/chainer,okuta/chainer,niboshi/chainer,okuta/chainer,keisuke-umezawa/chainer,chainer/chainer,wkentaro/chainer,chainer/chainer,chainer/chainer,jnishi/chainer,okuta/chainer,wkentaro/chainer,wkentaro/chainer,jnishi/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,jnishi/chainer,okuta/chainer,hvy/chainer,hvy/chainer