Add testing of links in docs

Depends on !164 (merged).

docs/Gemfile

@@ -19,6 +19,8 @@ gem "github-pages", "~> 215", group: :jekyll_plugins
   #gem "jekyll-feed", "~> 0.12"
 #end
 
+gem 'html-proofer'
+
 # Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem
 # and associated library.
 platforms :mingw, :x64_mingw, :mswin, :jruby do