Add an example about Comms

### 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.

Use the classic Notebook v6 comms page and the JupyterLab `kernel.registerCommTarget()` API as references. Add a dedicated JupyterLab Comms example and ensure it can be linked from the Notebook v7 and JupyterLab documentation.

Written by the indexing model from the issue text.