Add model accuracies to docstrings

[jneeven]

Feature motivation

Since the docs are maintained separately from the repo, it would be good to state the imagenet accuracy of each model (and potentially other stats such as number of params and model size) to the model docstring. This mitigates the risk of confusion and/or bugs should the docs ever become out of sync.

Feature description

Assuming the stats on https://docs.larq.dev/zoo/ are currently correct, we can just copy those into the relevant docstrings.