Add markdown lint action

Author: dflook

.config/.markdownlint.yaml

@@ -0,0 +1,8 @@
+line-length:
+  line_length: 200
+  tables: false
+no-inline-html:
+  allowed_elements: ['p', 'img']
+ul-style:
+  style: sublist
+

.config/changelog.markdownlint.yaml

@@ -0,0 +1,11 @@
+line-length:
+  line_length: 300
+no-inline-html:
+  allowed_elements: ['p', 'img']
+ul-style:
+  style: sublist
+no-duplicate-heading:
+  siblings_only: true
+blanks-around-headings:
+  lines_below: 0
+blanks-around-lists: false

.github/workflows/test.yaml

@@ -96,3 +96,21 @@ jobs:
           bash <(curl https://raw.githubusercontent.com/rhysd/actionlint/main/scripts/download-actionlint.bash)
           ./actionlint
           ./actionlint example_workflows/*.yaml
+
+      - name: Lint READMEs
+        uses: DavidAnson/markdownlint-cli2-action@v19
+        with:
+          config: '.config/.markdownlint.jsonc'
+          globs: '**/README.md'
+
+      - name: Lint CHANGELOG
+        uses: DavidAnson/markdownlint-cli2-action@v19
+        with:
+          config: '.config/.markdownlint.jsonc'
+          globs: 'CHANGELOG.md'
+
+      - name: Lint Other Markdown
+        uses: DavidAnson/markdownlint-cli2-action@v19
+        with:
+          config: '.config/.markdownlint.jsonc'
+          globs: 'docs/*.md'