Fix broken Pages deploy and gate PRs with a build

.github/workflows/ci.yml

@@ -0,0 +1,41 @@
+name: CI
+
+on:
+  pull_request:
+    branches: [main]
+  workflow_dispatch:
+
+# A newer push to the same PR makes the in-flight run pointless.
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    name: Build docs
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 24
+          cache: pnpm
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      # VitePress fails the build on dead internal links, so this also
+      # catches links to pages that do not exist.
+      - name: Build
+        run: pnpm docs:build

pnpm-workspace.yaml

@@ -1,2 +1,5 @@
+packages:
+  - .
+
 allowBuilds:
   esbuild: true