Demonstrate markdown linting working correctly

.github/actions/lint/action.yml

@@ -44,3 +44,6 @@ runs:
     - name: Check formatting
       shell: bash
       run: yarn run format-check
+    - name: Lint markdown
+      shell: bash
+      run: yarn run lint-markdown

.github/actions/yarn-install/action.yml

@@ -9,7 +9,7 @@ runs:
         ATTEMPT=0
         WAIT_TIME=20
         while [ $ATTEMPT -lt $MAX_ATTEMPTS ]; do
-            yarn install --non-interactive --frozen-lockfile && break
+            yarn install --non-interactive --frozen-lockfile --ignore-engines && break
             echo "yarn install failed. Retrying in $WAIT_TIME seconds..."
             sleep $WAIT_TIME
             ATTEMPT=$((ATTEMPT + 1))

.markdownlint-cli2.mjs

@@ -0,0 +1,42 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import relativeLinksRule from 'markdownlint-rule-relative-links';
+
+const config = {
+  config: {
+    'heading-increment': true,
+    'no-reversed-links': true,
+    'no-missing-space-atx': true,
+    'no-duplicate-heading': {
+      siblings_only: true,
+    },
+    'single-title': true,
+    'no-trailing-punctuation': true,
+    'no-space-in-emphasis': true,
+    'no-space-in-code': true,
+    'no-space-in-links': true,
+    'fenced-code-language': true,
+    'first-line-heading': true,
+    'no-empty-links': true,
+    'no-alt-text': true,
+    'link-fragments': true,
+    'table-column-count': true,
+
+    // The rest of default rules are already handled by prettier
+
+    // Custom rules
+    'relative-links': true,
+  },
+  globs: ['**/__docs__/*.md'],
+  ignores: ['**/node_modules'],
+  customRules: [relativeLinksRule],
+};
+
+export default config;

__docs__/README.md

@@ -12,6 +12,8 @@ Native and contribute to it. **End users of React Native are meant to use the
 For details on how we approach technical documentation in this repository, see
 [GUIDELINES.md](./GUIDELINES.md).
 
+[This link does not exist!](./doesnotexist.md).
+
 ## Usage
 
 This repository is not meant to be consumed directly by end users. Instead, it

package.json

@@ -18,6 +18,7 @@
     "featureflags": "yarn --cwd packages/react-native featureflags",
     "lint-ci": "./.github/workflow-scripts/analyze_code.sh && yarn shellcheck",
     "lint-java": "node ./scripts/lint-java.js",
+    "lint-markdown": "markdownlint-cli2 2>&1",
     "lint": "eslint .",
     "prettier": "prettier --write \"./**/*.{js,md,yml,ts,tsx}\"",
     "print-packages": "node ./scripts/monorepo/print",
@@ -90,6 +91,8 @@
     "jest-diff": "^29.7.0",
     "jest-junit": "^10.0.0",
     "jest-snapshot": "^29.7.0",
+    "markdownlint-cli2": "^0.17.2",
+    "markdownlint-rule-relative-links": "^4.0.1",
     "metro-babel-register": "^0.82.0",
     "metro-memory-fs": "^0.82.0",
     "metro-transform-plugins": "^0.82.0",