docs: check links with html-proofer

This can be done with `bundle exec rake test`.

docs/.ruby-version

@@ -1 +1 @@
-2.4.1
+2.5.1

docs/Gemfile

@@ -1,3 +1,8 @@
 source "https://rubygems.org"
 
 gem "github-pages", group: :jekyll_plugins
+
+group :test do
+  gem "html-proofer"
+  gem "rake"
+end

docs/Gemfile.lock

@@ -13,6 +13,7 @@ GEM
       execjs
     coffee-script-source (1.11.1)
     colorator (1.1.0)
+    colorize (0.8.1)
     commonmarker (0.17.13)
       ruby-enum (~> 0.5)
     concurrent-ruby (1.0.5)
@@ -84,6 +85,15 @@ GEM
     html-pipeline (2.8.4)
       activesupport (>= 2)
       nokogiri (>= 1.4)
+    html-proofer (3.9.2)
+      activesupport (>= 4.2, < 6.0)
+      addressable (~> 2.3)
+      colorize (~> 0.8)
+      mercenary (~> 0.3.2)
+      nokogiri (~> 1.8.1)
+      parallel (~> 1.3)
+      typhoeus (~> 1.3)
+      yell (~> 2.0)
     http_parser.rb (0.6.0)
     i18n (0.9.5)
       concurrent-ruby (~> 1.0)
@@ -209,9 +219,11 @@ GEM
       mini_portile2 (~> 2.3.0)
     octokit (4.12.0)
       sawyer (~> 0.8.0, >= 0.5.3)
+    parallel (1.12.1)
     pathutil (0.16.1)
       forwardable-extended (~> 2.6)
     public_suffix (2.0.5)
+    rake (12.3.1)
     rb-fsevent (0.10.3)
     rb-inotify (0.9.10)
       ffi (>= 0.5.0, < 2)
@@ -237,12 +249,15 @@ GEM
     tzinfo (1.2.5)
       thread_safe (~> 0.1)
     unicode-display_width (1.4.0)
+    yell (2.0.7)
 
 PLATFORMS
   ruby
 
 DEPENDENCIES
   github-pages
+  html-proofer
+  rake
 
 BUNDLED WITH
-   1.16.1
+   1.16.4

docs/Rakefile

@@ -0,0 +1,31 @@
+require "rake"
+
+task default: :build
+
+desc "Build the site."
+task :build do
+  sh "jekyll", "build"
+end
+
+desc "Run html proofer to validate the HTML output."
+task test: :build do
+  require "html-proofer"
+  HTMLProofer.check_directory(
+    "./_site",
+    parallel: { in_threads: 4 },
+    favicon: true,
+    http_status_ignore: [0, 403],
+    assume_extension: true,
+    check_favicon: true,
+    check_opengraph: true,
+    check_html: true,
+    check_img_http: true,
+    file_ignore: [
+      %r{Kickstarter-Supporters},
+    ],
+    url_ignore: [
+      "http://formulae.brew.sh",
+      %r{https://github.com/Homebrew/brew/edit/gh-pages//},
+    ]
+  ).run
+end