add markdownlint config and pre-commit hook

.markdownlint.yml

@@ -0,0 +1,11 @@
+MD013:
+  line_length: 88
+  heading_line_length: 88
+  code_block_line_length: 100
+
+MD024:
+  siblings_only: true
+
+MD053:
+  ignored_definitions:
+    - "unreleased"

.pre-commit-config.yaml

@@ -60,6 +60,10 @@ repos:
         language: system
         types: [text]
         stages: [commit, push, manual]
+  - repo: https://github.com/igorshubovych/markdownlint-cli
+    rev: v0.38.0
+    hooks:
+    - id: markdownlint
   - repo: https://github.com/pre-commit/mirrors-prettier
     rev: v2.6.0
     hooks: