Add github action to lint unmerged docs

.github/workflows/build.yml

@@ -137,3 +137,49 @@ jobs:
         with:
           command: clippy
           args: -- -D warnings
+
+  docs:
+    name: Docs
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v2
+        with:
+          submodules: true
+
+      - name: Install stable toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+
+      - name: Cache cargo registry
+        uses: actions/cache@v2.1.6
+        with:
+          path: ~/.cargo/registry
+          key: ${{ runner.os }}-v2-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Cache cargo index
+        uses: actions/cache@v2.1.6
+        with:
+          path: ~/.cargo/git
+          key: ${{ runner.os }}-v2-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Cache cargo target dir
+        uses: actions/cache@v2.1.6
+        with:
+          path: target
+          key: ${{ runner.os }}-v2-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Generate docs
+        uses: actions-rs/cargo@v1
+        with:
+          command: xtask
+          args: docgen
+
+      - name: Check uncommitted documentation changes
+        run: |
+          git diff-files --quiet \
+            || echo "Run 'cargo xtask docgen', commit the changes and push again" \
+            && exit 1