.gitlab-ci.yml: Next try to install pandoc, now with conda.

.gitlab-ci.yml

 pages:
-  image:
-    name: pandoc
-    entrypoint: ["/bin/sh", "-c"]
-
   script:
   - cd doc
+  - conda install -c conda-forge pandoc
   - pip install nbsphinx nbsphinx_link pydata_sphinx_theme
   - make html
   - mv _build/html/* ../public/