Add an example about Comms

[jtpio]

Problem

The classic notebook v6 documentation has a dedicated page about Comms: https://jupyter-notebook.readthedocs.io/en/v6.4.8/comms.html

They include JavaScript code snippets to define comm targets on the frontend.

But the JupyterLab documentation doesn't appear to have anything like this, using the kernel.registerCommTarget() API.

Proposed Solution

A dedicated example would be great, and could be linked to from the Notebook v7 and JupyterLab docs.