add python environment customization guide

[mbektas]

adding instructions to guide on how to set proper permissions and customize the python environment

[krassowski]

It looks good to me! I would consider adding:

• an info on re-using existing kernels by providing a custom kernelspec (see Jupyter Server Not Found (executable not in PATH) #238 (comment) but I don't know if these steps work for others yet)
• an information on when users should customise the virtual environment (e.g. installing server extensions, or using jupyterlab-app just for a single project), and when to re-use other kernels instead (which I would recommended when there are multiple virtual environments.projects)

[mbektas]

It looks good to me! I would consider adding:

• an info on re-using existing kernels by providing a custom kernelspec (see #238 (comment) but I don't know if these steps work for others yet)
• an information on when users should customise the virtual environment (e.g. installing server extensions, or using jupyterlab-app just for a single project), and when to re-use other kernels instead (which I would recommended when there are multiple virtual environments.projects)

thanks @krassowski!, sure, lets extend the guide with follow-up PRs.