Also pip install the notebook for building docs via conda env

To build the docs you need the notebook package installed from
the local project so this updates the contributor guide for the
conda instructions.

Related #5741

Author: mriedem

CONTRIBUTING.rst

@@ -176,7 +176,8 @@ containing all the necessary packages (except pandoc), use::
 
     conda env create -f docs/environment.yml
     conda activate notebook_docs  # Linux and OS X
-    activate notebook_docs         # Windows
+    activate notebook_docs        # Windows
+    pip install .
 
 .. _conda environment:
     https://conda.io/docs/user-guide/tasks/manage-environments.html#creating-an-environment-from-an-environment-yml-file