CI: Add Github workflow

.github/workflows/main.yml

@@ -0,0 +1,30 @@
+name: Build docs and check links
+on: [push, pull_request]
+jobs:
+  docs:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Clone repo
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+        submodules: true
+    - name: Set up Python
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.8
+    - name: Double-check Python version
+      run: |
+        python --version
+    - name: Install Python package
+      run: |
+        python -m pip install .
+    - name: Install docs dependencies
+      run: |
+        python -m pip install -r doc/requirements.txt
+    - name: Build HTML
+      run: |
+        python -m sphinx doc/ _build/html/ -d _doctrees/ -W -b html
+    - name: Check links
+      run: |
+        python -m sphinx doc/ _build/linkcheck/ -d _doctrees/ -W -b linkcheck