add doxygen to the build

.github/workflows/idefix-ci-doc.yml

@@ -20,6 +20,7 @@ jobs:
         with:
           submodules: recursive
       - name: install dependencies
+        run: sudo apt-get install -y doxygen
         run: python -m pip install --exists-action=w --no-cache-dir -r doc/python_requirements.txt
       - name: compile documentation
         run: python -m sphinx -T -b html -d _build/doctrees -D language=en doc/source doc/html