CI: check documentation (#1246)

.github/workflows/ci.yml

@@ -160,6 +160,29 @@ jobs:
           command: test
           args: ${{ matrix.features }} ${{ matrix.test-features }} -- --test-threads=1
 
+  docs:
+    name: Docs
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Install Rust
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          profile: minimal
+          override: true
+
+      - name: Check documentation
+        env:
+          RUSTDOCFLAGS: -D warnings
+        uses: actions-rs/cargo@v1
+        with:
+          command: doc
+          args: --no-deps --document-private-items --all-features
+
   # Separate build job for nightly because of the missing feature for allowed failures at
   # job level. See `jobs.build.strategy.matrix`.
   nightly: