Add an example for creating a new IDrive

[jtpio]

Problem

Custom drives are a powerful feature of JupyterLab that allow extensions to provide additional sources of contents.

For example:

• https://github.com/jupyterlab-contrib/jupyterlab-filesystem-access
• https://github.com/jupyterlab/jupyterlab-github

Proposed Solution

Creating a new drive consists in implementing the Contents.IDrive interface and should not be too complex depending on where the contents get stored of course.

Maybe there should be new example showing how to do this, for example for storing notebooks and files in the browser local storage (localStorage, IndexedDB).

Additional context

[jtpio]

Actually even the JupyterLab does not seem to be mentioning it in the common extension points: https://jupyterlab.readthedocs.io/en/latest/extension/extension_points.html

[timkpaine]

Writing examples would also probably be a good opportunity to see if there's any improvements that are immediately relevant to do on the server side, this has been the main impediment of https://github.com/jpmorganchase/jupyter-fs.