Markdown entered by one user does not render for other users in the same session

[ibdafna]

In a collaborative session, when user A inserts a new markdown cell, types some content in it and "executes" it so it renders, it only renders for user A, but not for all other users in the session. If user A inserts a code cell first, types in it, and then converts it to markdown cell and executes it - the markdown gets rendered for everyone in the session.

(demo towards the end of the animated gif)

[davidbrochart]

The information about whether the Markdown cell is rendered or not is not part of the notebook format, nor the notebook shared model, it's purely a frontend thing that allows to edit a Markdown cell. But we could maybe include that information in the notebook shared model. There are discussions about including the cell execution state in https://github.com/jupyter-server/jupyter_ydoc/pull/197, which currently only concerns code cells. Maybe this could be more general and apply to Markdown cells too? For instance if we rename the possible values to rendered/rendering instead of idle/busy. What do you think?