The JupyterLab extension for Jupytext should let the user know that a server extension is required

[mwouts]

When installing the git extension for Jupyterlab, the user gets this message:

We should have the same for Jupytext.

Additionally, the current name of the jupyterlab-jupytext is A JupyterLab extension for Jupytext which does not tell much about what it does, we should have a better name that could include: keywords like: git, version control, export, text, script, Markdown...