Explore one-to-many relationship with Voila template

[jess-x]

Is your feature request related to a problem? Please describe.

From my understanding, Voila template has one-to-one relationship to notebook. Interactive-dashboard now has the ability to drag and drop outputs and markdown from multiple notebooks.

Describe the solution you'd like

Explore a one-to-many relationship with voila template

Additional context

The .dashboard file format now stores the widget info from multiple notebooks, and we are also planning to work on the relinking of missing notebook when opening a .dashboard file inside jupyterlab