Add jekyll-simple-i18n plugin with example

Adds in support for https://github.com/signalapp/jekyll-simple-i18n.

The plugin did not work with the github-pages gem, so github-pages was
switched with jekyll (which is what the current i18n branch does anyway).

Gemfile

@@ -1,11 +1,12 @@
 source "https://rubygems.org"
 
 # Install github-pages gem (https://jekyllrb.com/docs/github-pages/)
-gem "github-pages", group: :jekyll_plugins
+gem "jekyll", group: :jekyll_plugins
 
 # Plugins
 group :jekyll_plugins do
   # example: gem "jekyll-feed", "~> 0.6"
+  gem "jekyll-sitemap"
 end
 
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem