Add help text to the widget pointing users to helpful info about customizations when the cluster is instantiated from the extension

Instantiating a new cluster from dask-labextension is a really neat future. However, for users who are not familiar with configuring dask settings from configuration files, knowing how to apply custom settings (such as memory, cores, processes) is not trivial unless you know that there's a configuration file for this residing in `~/.config/dask/labextension.yaml`.

Would it be useful to add some help text to the widget telling the user the location of the configuration file, and maybe a pointer to the documentation page?


Cc @lheagy