chore: Add Markdownlint to CI (#222)

- Turn off failing linter rules
- Add problem matcher to show errors in CI results
- Limit CI run to Markdown changes

Author: nschonni

.github/workflows/markdownlint-problem-matcher.json

@@ -0,0 +1,17 @@
+{
+    "problemMatcher": [
+        {
+            "owner": "markdownlint",
+            "pattern": [
+                {
+                    "regexp": "^([^:]*):(\\d+):?(\\d+)?\\s([\\w-\\/]*)\\s(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "column": 3,
+                    "code": 4,
+                    "message": 5
+                }
+            ]
+        }
+    ]
+}

.github/workflows/markdownlint.yml

@@ -0,0 +1,32 @@
+name: Markdownlint
+
+on:
+  push:
+    paths:
+      - "**/*.md"
+      - ".markdownlint.json"
+      - ".github/workflows/markdownlint.yml"
+      - ".github/workflows/markdownlint-problem-matcher.json"
+  pull_request:
+    paths:
+      - "**/*.md"
+      - ".markdownlint.json"
+      - ".github/workflows/markdownlint.yml"
+      - ".github/workflows/markdownlint-problem-matcher.json"
+
+jobs:
+  lint:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Use Node.js
+      uses: actions/setup-node@v1
+      with:
+        node-version: 12.x
+    - name: Run Markdownlint
+      run: |
+        echo "::add-matcher::.github/workflows/markdownlint-problem-matcher.json"
+        npm i -g markdownlint-cli
+        markdownlint "**/*.md"

.markdownlint.json

@@ -0,0 +1,22 @@
+{
+    "default": true,
+    "MD004": false,
+    "MD007": false,
+    "MD009": false,
+    "MD010": false,
+    "MD012": false,
+    "MD013": false,
+    "MD014": false,
+    "MD022": false,
+    "MD024": false,
+    "MD025": false,
+    "MD026": false,
+    "MD029": false,
+    "MD030": false,
+    "MD032": false,
+    "MD033": false,
+    "MD034": false,
+    "MD036": false,
+    "MD040": false,
+    "MD041": false
+}