.gitlab-ci.yml: Install pandoc with apt-get.

.gitlab-ci.yml

 pages:
   script:
   - cd doc
-  - pip install nbsphinx nbsphinx_link pydata_sphinx_theme pandoc
+  - pip install nbsphinx nbsphinx_link pydata_sphinx_theme
+  - apt-get install pandoc
   - make html
   - mv _build/html/* ../public/