add merlin6 JB docs

docs/merlin6/jupyterhub/jupyterlab.md

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+# Jupyterlab User interface
+
+## Testing out Jupyterlab
+
+**Jupyterlab** is a new interface to interact with your Jupyter notebooks.
+However, it is in very active development and undergoing constant changes. You
+can read about its features [on the official
+website](https://jupyterlab.readthedocs.io/en/stable/user/interface.html).
+
+You can test it out on our server by using the following kind of URL, where
+`$YOUR-USER` must be replaced by your PSI username. You must already have an
+active session on the jupyterhub.
+
+```text
+https://merlin-jupyter.psi.ch:8000/user/$YOUR-USER/lab
+```
+
+## Switching to the Classic Notebook user interface
+
+You can switch to the classical notebook UI by using the **"Launch Classic Notebook"** command from the left sidebar of JupyterLab.
+
+![Launch Classic Notebook](../../images/jupyter/jupyter-launch-classic.png)
+/// caption
+///
+
+## Jupyterlab does not support the older nbextensions
+
+These regrettably are not yet supported from within the JupyterLab UI,
+but you can activate them through the Classic Notebook interface (see
+above)