Add CI to new pull requests

Run a simple hugo build step on every pull request to ensure the website
remains healthy.

.github/workflows/CI.yml

@@ -0,0 +1,21 @@
+name: Check hugo build
+
+'on':
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  build-deploy:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+      with:
+        submodules: true
+        ssh-key: ''
+
+    - name: Setup Hugo
+      uses: peaceiris/actions-hugo@v2
+
+    - name: Build
+      run: hugo --minify