Add 1-click UX to fetch extension-examples without manual clone

> what is the UX when installed locally without the submodule? should it show instructions to clone the subfolder? Ideal UX would be 1-click but there are larger technical choices, so let's track this in a new issue 

_Originally posted by @krassowski in https://github.com/jupyterlab/plugin-playground/issues/86#issuecomment-4011745616_
            