Install yarn<3 in dev env (#1328)

Author: davidbrochart

docs/source/contribute.rst

@@ -31,7 +31,7 @@ First, you need to fork the project. Then setup your environment:
 .. code-block:: bash
 
    # create a new conda environment
-   conda create -n voila -c conda-forge notebook jupyterlab nodejs yarn pip
+   conda create -n voila -c conda-forge notebook jupyterlab nodejs "yarn<3" pip
    conda activate voila
 
    # download voila from your GitHub fork