docs: add mdl for Markdown linting

Run with `mdl -c docs/.mdlrc docs/*.md`.

docs/.mdl_ruleset.rb

@@ -0,0 +1,7 @@
+rule 'HB034', 'Bare unstyled URL used' do
+  tags :links, :url
+  aliases 'no-bare-unstyled-urls'
+  check do |doc|
+    doc.matching_text_element_lines(%r{(?<=\s)https?://})
+  end
+end

docs/.mdl_style.rb

@@ -0,0 +1,7 @@
+all
+rule 'MD026', punctuation: ',;:' # Trailing punctuation in header
+exclude_rule 'MD013' # Line length
+exclude_rule 'MD029' # Ordered list item prefix
+exclude_rule 'MD033' # Inline HTML
+exclude_rule 'MD034' # Bare URL used (replaced by HB034)
+exclude_rule 'MD046' # Code block style

docs/.mdlrc

@@ -0,0 +1,4 @@
+git_recurse true
+ignore_front_matter true
+style "#{File.dirname(__FILE__)}/.mdl_style.rb"
+rulesets ["#{File.dirname(__FILE__)}/.mdl_ruleset.rb"]

docs/Gemfile

@@ -4,5 +4,6 @@ gem "github-pages", group: :jekyll_plugins
 
 group :test do
   gem "html-proofer"
+  gem "mdl"
   gem "rake"
 end

docs/Gemfile.lock

@@ -9,6 +9,8 @@ GEM
       zeitwerk (~> 2.2, >= 2.2.2)
     addressable (2.8.0)
       public_suffix (>= 2.0.2, < 5.0)
+    chef-utils (17.8.25)
+      concurrent-ruby
     coffee-script (2.4.1)
       coffee-script-source
       execjs
@@ -226,6 +228,12 @@ GEM
     listen (3.7.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
+    mdl (0.11.0)
+      kramdown (~> 2.3)
+      kramdown-parser-gfm (~> 1.1)
+      mixlib-cli (~> 2.1, >= 2.1.1)
+      mixlib-config (>= 2.2.1, < 4)
+      mixlib-shellout
     mercenary (0.3.6)
     mini_portile2 (2.6.1)
     minima (2.5.1)
@@ -233,6 +241,11 @@ GEM
       jekyll-feed (~> 0.9)
       jekyll-seo-tag (~> 2.1)
     minitest (5.14.4)
+    mixlib-cli (2.1.8)
+    mixlib-config (3.0.9)
+      tomlrb
+    mixlib-shellout (3.2.5)
+      chef-utils
     multipart-post (2.1.1)
     nokogiri (1.12.5)
       mini_portile2 (~> 2.6.1)
@@ -272,6 +285,7 @@ GEM
     terminal-table (1.8.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     thread_safe (0.3.6)
+    tomlrb (2.0.1)
     typhoeus (1.4.0)
       ethon (>= 0.9.0)
     tzinfo (1.2.9)
@@ -289,7 +303,8 @@ PLATFORMS
 DEPENDENCIES
   github-pages
   html-proofer
+  mdl
   rake
 
 BUNDLED WITH
-   1.17.2
+   1.17.3

docs/Rakefile

@@ -7,7 +7,12 @@ task :build do
   sh "jekyll", "build"
 end
 
-desc "Run html proofer to validate the HTML output."
+desc "Run Markdownlint to validate the Markdown style."
+task lint: :build do
+  sh "mdl", "."
+end
+
+desc "Run HTMLProofer to validate the HTML output."
 task test: :build do
   require "html-proofer"
   HTMLProofer.check_directory(