ci: add markdownlint problem matcher (#49751)

.github/problem-matchers/markdownlint.json

@@ -0,0 +1,16 @@
+{
+  "problemMatcher": [
+    {
+      "owner": "markdownlint",
+      "pattern": [
+        {
+          "regexp": "^(.+):(\\d+):(\\d+)\\s+(.*)$",
+          "file": 1,
+          "line": 2,
+          "column": 3,
+          "message": 4
+        }
+      ]
+    }
+  ]
+}

.github/workflows/pipeline-electron-lint.yml

@@ -65,9 +65,11 @@ jobs:
         curl -sL "https://chromium.googlesource.com/chromium/src/+/${chromium_revision}/buildtools/DEPS?format=TEXT" | base64 -d > src/buildtools/DEPS
 
         gclient sync --spec="solutions=[{'name':'src/buildtools','url':None,'deps_file':'DEPS','custom_vars':{'process_deps':True},'managed':False}]"
-    - name: Add ESLint problem matcher
+    - name: Add problem matchers
       shell: bash
-      run: echo "::add-matcher::src/electron/.github/problem-matchers/eslint-stylish.json"
+      run: |
+        echo "::add-matcher::src/electron/.github/problem-matchers/eslint-stylish.json"
+        echo "::add-matcher::src/electron/.github/problem-matchers/markdownlint.json"
     - name: Run Lint
       shell: bash
       run: |