Document the new JupyterHub share UX

[mfisher87]

Problem

#438 is awesome! I'd love a documentation page about this feature and how to configure JH & JL to make it work.

Proposed Solution

A new page in the docs!

Additional context

I'd be happy to work on this, but would like some support about the specific details we need to add. @yuvipanda and I are working on setting up a Hub now, so I can also test the documentation as we go.

[Hyrla]

Hi,
Sorry I didn't take the time to document the share UI. I will try to do so in september (unless someone can do it earlier!). If you want to try this new UI before that, check my comment here: #438 (comment)