Add pip install of doc-requirements.txt

To be able to build the docs the libraries in docs/doc-requirements.txt
need to be installed. As such, the installation of these libraries
should be part of the setup of the developer virtual env.

CONTRIBUTING.md

@@ -75,6 +75,7 @@ python3 -m venv .
 source bin/activate
 pip3 install -e .
 pip3 install -r dev-requirements.txt
+pip3 install -r docs/doc-requirements.txt
 ```
 
 This should install all the libraries required for testing & running repo2docker!