updated gitlab-ci to install requirements before building the documentation....

I believe the problem with autodoc constructing the documentation was caused by missing dependencies. This merge request install all pyicon dependencies before attempting to build the documentation.

.gitlab-ci.yml

 pages:
   script:
-  - cd doc
   - apk add --repository https://dl-cdn.alpinelinux.org/alpine/edge/community pandoc
-  - pip install nbsphinx nbsphinx_link pydata_sphinx_theme
+  - pip install -r requirements.txt
+  - pip install cartopy
+  - pip install nbsphinx nbsphinx_link pydata_sphinx_theme sphinx_copybutton
+  - cd doc
   - make html
   - mv _build/html/* ../public/