Add GitHub build step for documentation

.github/workflows/build.yml

@@ -0,0 +1,19 @@
+name: build
+
+on:
+  push:
+    branches: ["test-ci"]
+
+jobs:
+  build-docs:
+    runs-on: ubuntu-latest
+    container: riot/riotbuild
+    steps:
+      - name: Check out rust-documentation-builder
+        uses: actions/checkout@v3
+      - name: Run `make build-from-checkout`
+        # Note that this runs the RIOT checkout internally, both to reduce
+        # dependencies on GitHub actions and to gain the flexibility on running
+        # with different sources with the same ease locally and in CI.
+        run: make build-from-checkout
+