Add markdown linting to GitHub workflow

Commit introduces initial set of rules for checking formatting of
markdown files as part of GitHub workflow. The rules are:

  - first line is top-level heading
  - there is only one top-level heading in the document

.copy-edit-stylesheet-checklist.md

@@ -1,7 +1,11 @@
 # Basic checklist/stylesheet used for copy editing BOLTs
 
 Contributions should comply with this checklist/stylesheet to maintain correct, clear, consistent, and concise BOLTS. 
+Items marked as 'lint' can be automatically checked by running `tools/lint.sh`.
 
+  - headings
+    - first line is top-level heading (lint)
+    - there is only one top-level heading in the document (lint)
   - spelling
     - run `tools/spellcheck.sh --check [0-9][0-9]-*.md`
       - update `.aspell.en.pws` with any missing words

.github/workflows/checks.yml

@@ -0,0 +1,12 @@
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: DavidAnson/markdownlint-cli2-action@v15
+      with:
+        globs: |
+          ??-*.md
+          proposals/*.md

.markdownlint.jsonc

@@ -0,0 +1,9 @@
+{
+  "default": false,
+
+  // There should be only one top-level heading.
+  "MD025": true,
+
+  // The first line should be top-level heading.
+  "MD041": true
+}

tools/lint.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+# https://github.com/DavidAnson/markdownlint-cli2
+
+markdownlint-cli2 ??-*.md proposals/*.md