ci: add markdownlint check

.github/workflows/markdownlint.yml

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+name: Markdownlint check
+
+on:
+  push:
+    branches: [main]
+    paths: ["**.md"]
+  pull_request:
+
+jobs:
+  markdownlint:
+    name: Markdownlint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DavidAnson/markdownlint-cli2-action@v18
+        with:
+          globs: "**/*.md"

.markdownlint.yaml

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+# Defaults https://github.com/DavidAnson/markdownlint/blob/main/schema/.markdownlint.yaml
+# DOCS https://github.com/markdownlint/markdownlint/blob/main/docs/RULES.md
+#───────────────────────────────────────────────────────────────────────────────
+
+# MODIFIED SETTINGS
+blanks-around-headings:
+  lines_below: 0 # space waster
+ul-style: { style: sublist }
+
+# not autofixable
+ol-prefix: { style: ordered }
+line-length:
+  tables: false
+  code_blocks: false
+no-inline-html:
+  allowed_elements: [img, details, summary, kbd, a, br]
+
+#─────────────────────────────────────────────────────────────────────────────
+# DISABLED
+ul-indent: false # not compatible with using tabs
+no-hard-tabs: false # taken care of by editorconfig
+blanks-around-lists: false # space waster
+first-line-heading: false # e.g., ignore-comments
+no-emphasis-as-heading: false # sometimes useful