Check markdown links in CI

.github/workflows/ci.yml

@@ -0,0 +1,15 @@
+name: CI
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  xrefcheck:
+    name: Check references
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: serokell/xrefcheck-action@v1
+
+# TODO: Use https://github.com/marketplace/actions/github-codeowners-validator