Use Github Actions for Doxygen

.github/workflows/main.yml

@@ -0,0 +1,25 @@
+name: Documentation
+
+on:
+  push:
+    branches:
+    - master
+
+jobs:
+  build-deploy:
+    runs-on: ubuntu-18.04
+    steps:
+    - uses: actions/checkout@master
+
+    - name: build
+      run: |
+        apt -y install doxygen
+        cd doc
+        doxygen ./Doxyfile
+
+    - name: deploy
+      uses: peaceiris/actions-gh-pages@v2.2.0
+      env:
+        ACTIONS_DEPLOY_KEY: ${{ secrets.ACTIONS_DEPLOY_KEY }}
+        PUBLISH_BRANCH: gh-pages
+        PUBLISH_DIR: ./doc/html