Hybrid content manager by default.

I would propose for (5.0, maybe earlier)
to bake into the notebook a notion of Hybrid content manager and "mount point" or the dashboard.

The would allow to have content manager that expose in some folder "normals" ipynbs, in others `ipymd`...etc.

By default, "normal" content manager would be mounted on `/`.

I want that to have **some** notebook as `ipymd` for collaboration.