Add markdown linter

Author: arnested

.github/workflows/markdown.yml

@@ -0,0 +1,26 @@
+---
+name: Markdown
+on: pull_request
+
+# Detect if this action is already running, and cancel it.
+# This most likely happened because a second push has been made to a branch.
+concurrency:
+  group: ${{ github.repository_id }}-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  markdown:
+    name: Markdownlint
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+      - name: Run markdownlint
+        uses: DavidAnson/markdownlint-cli2-action@v20
+        with:
+          globs: |
+            *.md
+            docs/*.md