ci: add actions

.github/workflows/mdlint.yaml

@@ -0,0 +1,21 @@
+name: Check Markdown
+on:
+  pull_request
+
+jobs:
+  check-markdown:
+    runs-on: ubuntu-22:04
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v2
+        with:
+          node-version: '21'
+
+      - name: Install mdlint
+        run: npm install -g @moonbit/markdown-linter
+
+      - name: Run mdlint
+        run: mdlint course*/*.md