Create a set of example notebooks

[jtpio]

These example notebooks could be added to the local storage by default (so available in the file browser on the first startup), and be used to quickly showcase a specific feature of a kernel or the language.

[jtpio]

For now maybe the simplest would be to have:

• a new examples-extension package, with a plugin that depends on the IContents service
• a set of notebooks in packages/examples-extension/notebooks, as normal .ipynb files with the correct metadata already populated (so it starts the right kernel)
• on build, notebooks are placed under app/examples/notebooks as-is
• on plugin activation, @jupyterlite/examples fetch the examples and adds them to the local storage with contents.save()
• to be less disruptive, they could even be placed under an examples/directory:

[bollwyvl]

This is the part where folk will want to be able to put their stuff in and all but _instantly_ get a working site, with notebooks, file assets, data sets, that work with markdown, image viewers, etc. I see this as realizing the /api/contents route, even if the _actual_ contents stay in their native format. As hinted at on #41, I think the part where one adds contents should definitely not require webpack, nor should examples be shipped as part of the distributed base assets. Really just should need to build up the JSON tree, which should be doable in node or python (or whatever).