Create markdown tools workflow

We want to be able to manually run MarkdownLint on CI.

[1]: https://github.com/DavidAnson/markdownlint
[2]: https://github.com/igorshubovych/markdownlint-cli

.github/workflows/markdown_tools.yaml

@@ -0,0 +1,29 @@
+name: Markdown tools
+on:
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  markdownlint:
+    name: MarkdownLint
+
+    permissions:
+      contents: read
+      pull-requests: read
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+      - name: Install Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 18
+      - name: Install MarkdownLint
+        run: npm install -g markdownlint-cli
+      - name: Checking markdown
+        run: markdownlint .