Add markdownlint config and github action

Uses rules from Redocly/marketing-site-portal#1142

.github/workflows/markdownlint.yml

@@ -0,0 +1,17 @@
+name: Markdown lint
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+
+jobs:
+  markdownlint:
+    name: markdownlint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: articulate/actions-markdownlint@v1
+        with:
+          config: markdownlint-config.json
+          files: 'docs/**/*.md'
+          ignore: node_modules
+          version: 0.28.1

.markdownlint.yaml

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+---
+# Default rules: https://github.com/github/super-linter/blob/master/TEMPLATES/.markdown-lint.yml
+
+# Rules by id
+
+# Unordered list style
+MD004: false
+
+# Unordered list indentation
+MD007:
+  indent: 2
+
+MD013:
+  # TODO: Consider to decrease allowed line length
+  line_length: 800
+  tables: false
+
+## Allow same headers in siblings
+MD024:
+  siblings_only: true
+
+# Multiple top level headings in the same document
+MD025:
+  front_matter_title: ''
+
+# Trailing punctuation in heading
+MD026:
+  punctuation: '.,;:。，；:'
+
+# Ordered list item prefix
+MD029: false
+
+# Unordered lists inside of ordered lists
+MD030: false
+
+# Inline HTML
+MD033: false
+
+# No bare urls
+MD034: false
+
+# Emphasis used instead of a heading
+MD036: false
+
+# Disable "First line in file should be a top level heading"
+# We use uncommon format to add metadata.
+# TODO: Consider to use "YAML front matter".
+MD041: false
+
+# Rules by tags
+blank_lines: false
+
+MD046: false
+# code-block-style