DOC: how to create separate conda environments on the jupyterhub

[scottyhq]

users always ask how to create a separate conda environment from the default environment that can be used to execute notebooks. You need to register the 'kernel'.

An example, and note that you might have to log out and log back into the server:

conda create -q -y -n isce2 python=3.8 isce2=2.5.1 ipykernel
conda activate isce2
# The following command is necessary for JupyterHub to find this kernel for your notebook
python -m ipykernel install --user --name isce2 --display-name "Python (isce2)"