add merlin6 JB docs

docs/merlin6/jupyterhub/jupyter-extensions.md

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+# Jupyter Extensions
+
+## Using nbextensions for adding features to your notebook
+
+There exist a number of useful [contributed but unofficial
+extensions](https://jupyter-contrib-nbextensions.readthedocs.io/en/latest/index.html)
+that add useful features to your notebooks.
+
+From the classic Notebook UI you can access the available extensions in a
+separate tab as displayed in the screenshot, below. You may have to unselect
+the *disable configuration for nbextensions without explicit copatibility*. The
+extensions we tested still worked fine with this jupyterhub version of 1.0.0.
+
+![Launch Classic Notebook](../../images/jupyter/jupyter-launch-classic.png)
+/// caption
+///
+
+## Extensions for working with large notebooks
+
+Especially the following extensions make working with larger notebooks easier
+
+* **Table of Contents**: Displays a TOC on the left and you can also configure it
+  to add and update a TOC at the head of the document.
+* **Collapsible Headings**: allows you to fold all the cells below a heading
+
+It may also be interesting for you to explore the [Jupytext](jupytext.html) server extension.
+
+## Variable Inspector
+
+The `variable inspector` extension provides a constantly updated window in which you can see the value and type of your notebook's variables.