fix(docs) install doxygen for GitHub Actions

.github/workflows/compile_docs.yml

@@ -13,6 +13,8 @@ jobs:
         uses: actions/setup-python@v1
         with:
           python-version: 3.7
+      - name: Install Doxygen
+        run: sudo apt-get install doxygen
       - name: Install requirements
         run: |
           pip install --upgrade --upgrade-strategy eager sphinx recommonmark commonmark breathe sphinx-rtd-theme sphinx-markdown-tables sphinx-sitemap