Add a markdown lint workflow

Author: bbatsov

.github/workflows/markdown-lint.yml

@@ -0,0 +1,21 @@
+name: Markdown Lint
+
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
+    - uses: tj-actions/changed-files@v45
+      id: changed-files
+      with:
+        files: '**/*.md'
+        separator: ","
+    - uses: DavidAnson/markdownlint-cli2-action@v19
+      if: steps.changed-files.outputs.any_changed == 'true'
+      with:
+        globs: ${{ steps.changed-files.outputs.all_changed_files }}
+        separator: ","