Here's what we do in geoadmin3:

https://github.com/geoadmin/mf-geoadmin3/blob/master/Makefile#L85

Both approaches are possible and I don't have a preference. With the submodule approach in the static folder, the whole git repository will be reachable in the browser. Without the submodule, our repository will get bigger.

In any case, we should aim to do things the same way if possible.