markup: add jupyter notebook markup language

Follows documentation here:
  - https://docs.gitea.io/en-us/external-renderers/#installing-external-binaries

This adds jupyter and configures gitea to use jupyter to render notebook files.

Dockerfile

@@ -4,6 +4,7 @@ RUN apt-get update && \
     apt-get install -y openssh-server git && \
     rm -rf /etc/ssh_host_* && \
     rm -r /var/cache/apt /var/lib/apt/lists
+RUN pip3 install jupyter
 
 ADD supervisor/ /etc/supervisor/conf.d/