doc: Verify Antora builds

Although we don't actively use Antora in Cockpit, our website docs will
depend on it working. If the GitHub workflow fails it is likely
something we need to address before merging to ensure that Cockpit Docs
functions as expected.

Signed-off-by: Freya Gustavsson <freya@venefilyn.se>

.github/workflows/verify-antora.yml

@@ -0,0 +1,32 @@
+name: verify-antora
+on:
+  pull_request:
+    # Only run when a file has changed in doc/ directory
+    paths:
+      - doc/**
+    branches:
+      - main
+jobs:
+  verify-antora:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Setup PNPM
+        uses: pnpm/action-setup@v4
+        with:
+            version: 10
+
+      - name: Setup Node with Cache
+        uses: actions/setup-node@v6
+        with:
+          node-version: 'latest'
+          cache: 'pnpm'
+          cache-dependency-path: '**/.github/workflows/*.yml'
+
+      - name: Verify Antora can build
+        run: pnpx antora antora-playbook.yml
+
+      - name: Show generated Antora files
+        run: tree doc/output/antora/ -h