Revise text-based markdown editor, remove associated side panel

[guerler]

Requires #19775. This PR sets the cell-based Markdown editor as the default and removes the previously required Markdown side panel.

How to test the changes?

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
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[jmchilton]

I think the document-based editor should remain the default paradigm and should be invested in. I think the goal should organic documents and not ad-hoc Jupyter-lite. I really didn't understand this was the goal when you added the cell editor.

[guerler]

Ok, closing this for now, and leaving the text-based editor as is with it's panel.

[guerler]

@jmchilton yes the goal was to find a better solution for the side panel with markdown items which are inserted at the last cursor position, another goal was to introduce previews. I am totally ok, with leaving the document-based editor as default even if we remove the side panel at one point.