Check for dartdoc warnings in GitHub Actions

.github/workflows/ci.yml

@@ -127,10 +127,23 @@ jobs:
       - name: Analyze dart
         run: dartanalyzer --fatal-warnings --fatal-infos lib tool test
 
+  dartdoc:
+    name: Dartdoc
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dart-lang/setup-dart@v1
+      - run: dart pub get
+      - name: Run dartdoc
+        run: dartdoc --quiet --no-generate-docs
+          --errors ambiguous-doc-reference,broken-link,deprecated
+          --errors unknown-directive,unknown-macro,unresolved-doc-reference
+
   sanity_checks:
     name: Sanity checks
     runs-on: ubuntu-latest
-    needs: [sass_spec, dart_tests, node_tests, static_analysis]
+    needs: [sass_spec, dart_tests, node_tests, static_analysis, dartdoc]
     if: "startsWith(github.ref, 'refs/tags/') && github.repository == 'sass/dart-sass'"
 
     steps: