add markdownlint to CI

.forgejo/workflows/check.yml

@@ -7,4 +7,8 @@ jobs:
     name: check
     steps:
       - uses: actions/checkout@v4
+      - uses: actions/markdownlint-cli2-action@v17
+        with:
+          globs: |
+            **/*.md
       - run: shellcheck *.sh monitoring/*.sh

.markdownlint.json

@@ -0,0 +1,3 @@
+{
+    "MD029": { "style": "zero" }
+}