use GitHub action workflow to check Markdown

.github/workflows/ci.yml

@@ -0,0 +1,38 @@
+name: Jekyll site CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup Ruby
+      uses: ruby/setup-ruby@v1.38.0
+      with:
+        ruby-version: 2.7
+    - name: Cache Ruby deps
+      uses: actions/cache@v2
+      with:
+        path: vendor/bundle
+        key: ruby-deps-${{ hashFiles('**/Gemfile.lock') }}
+        restore-keys: |
+          ruby-deps-
+    - name: Ruby deps
+      run: gem install json kramdown jekyll bundler
+    - name: Setup Python
+      uses: actions/setup-python@v2
+      with:
+        python-version: '3.x'
+    - name: Python deps
+      run: pip install pyyaml
+    - name: Nokogiri deps
+      run: sudo apt install -y pkg-config libxml2-dev libxslt-dev
+    - name: Site deps
+      run: bundle install
+    - name: Build site
+      run: bundle exec jekyll build
+    - name: Check all lessons
+      run: make lesson-check-all

Makefile

@@ -132,7 +132,7 @@ lesson-check : lesson-fixme
 
 ## * lesson-check-all : validate lesson Markdown, checking line lengths and trailing whitespace
 lesson-check-all :
-	@${PYTHON} bin/lesson_check.py -s . -p ${PARSER} -r _includes/links.md -l -w --permissive
+	@${PYTHON} bin/lesson_check.py -s . -p ${PARSER} -r _includes/links.md -l -w
 
 ## * unittest         : run unit tests on checking tools
 unittest :