Design doc: how to support a new device/library doc

[QiJune]

Fix #6576

[abhinavarora]

LGTM!
There are some portions where the language in the documentation needs to be polished. I am approving this PR and then I can begin a new PR that polishes the language and the grammar.

[helinwang]

Very informative, thank you @QiJune !

Curious what does groups do in:

template <typename DeviceContext, typename T>
class MaxOutFunctor {
 public:
  void operator()(const DeviceContext& context, const framework::Tensor& input,
                  framework::Tensor* output, int groups);
};