Merge pull request #6805 from krinsman/master

Redirect old tutorial to new tutorial

docs/source/developer/xkcd_extension_tutorial.rst

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+The extension tutorial has moved!
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+With the release of JupyterLab 1.0, we've also created a brand-new extension tutorial! This old tutorial is now out-of-date and has been retired, but `you can find the new tutorial here <https://jupyterlab.readthedocs.io/en/stable/developer/extension_tutorial.html>`__.