Cleanup Gemfile and add wdm on Windows (#2708)

docs/Gemfile

@@ -1,7 +1,4 @@
 source 'https://rubygems.org'
 
-gem "jekyll", "~> 3.8.5"
-
-gem "rouge"
-
-gem 'github-pages', group: :jekyll_plugins
+gem 'github-pages'
+gem 'wdm', '~> 0.1.1', :install_if => Gem.win_platform?