Build notebook distribution from contents of content/ directory

[psychemedia]

Relates to: https://github.com/jtpio/jupyterlite-demo/issues/2

Modify Github Action to:

• create all.json based on contents of content/ directory;
• copy contents of content/ directory into distribution.

[jtpio]

Thanks!

This will also be useful when we start implementing this kind of tooling in JupyterLite itself (https://github.com/jtpio/jupyterlite/issues/41)