Document development is now happening in jupyter/notebook repo

[jtpio]

Problem

Now that the RetroLab code base has been merged into the Jupyter Notebook repo: jupyter/notebook#6294

We should start documenting development will be happening over there.

Proposed Solution

• Add a section to the top of the readme to mention this: Add Notebook v7 notice to the README #357
• Transfer relevant issues to https://github.com/jupyter/notebook/
• Update issue templates in jupyterlab/retrolab to redirect to jupyter/notebook: Update issue templates to redirect to the notebook repo #364
• Archive the repo. This can wait until Notebook 7.0 final (or beta)

Additional context

Notebook v7 JEP: https://jupyter.org/enhancement-proposals/79-notebook-v7/notebook-v7.html

[jtpio]

Proceeding with the archival now to streamline the development over in https://github.com/jupyter/notebook/.

Feel free to open new issues or discussions in https://github.com/jupyter/notebook/ if you have any question, thanks!