CI: Build document on GitHub Actions

.github/workflows/builddoc.yml

@@ -0,0 +1,21 @@
+name: Build document
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.6
+    - name: Install dependencies
+      run:  |
+        sudo apt update
+        sudo apt install -y graphviz
+        pip install -U tox
+    - name: Run Tox
+      run:  tox -e docs