[CI] Add markdownlint workflow

Author: SaschaMann

.github/markdownlint.json

@@ -0,0 +1,6 @@
+{
+    "default": true,
+    "heading-style": "atx",
+    "line_length": false,
+    "no-inline-html": false
+}

.github/workflows/markdownlint.yml

@@ -0,0 +1,27 @@
+name: CI
+
+on:
+  push:
+    paths:
+      - "**.md"
+    branches:
+      - "main"
+  pull_request:
+    paths:
+      - "**.md"
+    branches:
+      - "main"
+
+jobs:
+  mdlint:
+    name: Lint Markdown
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f # 2.3.4
+
+      - name: Run markdownlint
+        uses: nosborn/github-action-markdown-cli@9fc95163471d6460c35cccad13645912aaa25d5f # 1.1.1
+        with:
+          files: "."
+          config_file: ".github/markdownlint.json"