Links between top-level deployment nodes in DeploymentView

Hi folks!

After a few hours of debugging to figure out why the links between my containers were not being shown in my deployment view, I saw https://github.com/goadesign/model/blob/2461d2ed73c9e307adfe0bc070b6950f82f56aac/expr/render.go#L101-L106

While it is true that you can technically add them using `Link`, it's not a great DX. Would you be interested on a PR adding something like `AddTopLevelLinks` (similar to the current `AddAll` and `AddDefault`) that is only applicable inside a `DeploymentView`?