Allow users to provide an external `ydoc` to `YDocument`

[trungleduc]

Problem

The constructor of YDocument is empty since the internal YJS doc is created by the class. It would be useful if users can provide an existing ydoc to the YDocument constructor.

Proposed Solution

Update the YDocument constructor signature to allow overriding the ydoc property.

Additional context

• This would help ypywidgets project to create frontend widgets.

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[davidbrochart]

This would also align with the Python side, where we can pass an existing YDoc.