Add GitHub CI

.github/workflows/main.yml

@@ -0,0 +1,20 @@
+name: CI
+on:
+  push:
+  pull_request:
+  merge_group:
+
+jobs:
+  build-docs:
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up PDM
+        uses: pdm-project/setup-pdm@v4
+        with:
+          python-version: 3.9
+
+      - name: Build docs
+        run: pdm run build