Mention Notebook 7 and other lab-based applications in the extension development documentation

[jtpio]

Problem

Currently the documentation for authoring extensions mostly focuses on JupyterLab extensions: extensions meant to be used within the default JupyterLab application shell: https://jupyterlab.readthedocs.io/en/latest/extension/extension_points.html#jupyter-front-end-shell

Notebook 7 leverages the existing ecosystem of JupyterLab extensions: https://jupyter.org/enhancement-proposals/79-notebook-v7/notebook-v7.html

Most extensions made for JupyterLab should also work with Notebook 7, although there are of course some differences between the two interfaces and shells.

Proposed Solution

We should mention there exists other lab-based applications and encourage extension authors to make extensions that will be compatible with them, not just with stock JupyterLab.

That could mean encouraging the use of optional dependencies, and explaining when using ILabShell is needed. But also mention there might be differences, for example an application might not have the command palette.

There could be a small section in the extension tutorial for example: https://jupyterlab.readthedocs.io/en/latest/extension/extension_tutorial.html. Maybe under an "Advanced" section, or on a dedicated docs page.

Additional context

This was discussed yesterday before and during the Notebook weekly call with @echarles @ericsnekbytes @RRosio @afshin: jupyter/notebook-team-compass#21 (comment)

[jtpio]

Closing as fixed by #14838