ci: add check workflow

.github/workflows/check.yaml

@@ -0,0 +1,22 @@
+name: Static analysis
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  static-analysis:
+    if: |
+      github.event_name == 'push' ||
+      github.event_name == 'pull_request' &&
+      github.event.pull_request.head.repo.full_name != github.repository
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup mandoc
+        run: sudo apt install -y mandoc
+
+      - run: make check