Move gh-pages branch files into /docs and add test files

- Jekyll ignore `/docs` and `/test` folders when using from root
- Update Staticman config to point to correct branch and data file location
- Replace `{{ base_path }}` references with `absolute_url` filter
- Update documentation

docs/Gemfile

@@ -0,0 +1,14 @@
+source "https://rubygems.org"
+
+gem "github-pages", group: :jekyll_plugins
+
+gem "wdm", "~> 0.1.0" if Gem.win_platform?
+
+# If you have any plugins, put them here!
+# group :jekyll_plugins do
+  # gem "jekyll-paginate"
+  # gem "jekyll-sitemap"
+  # gem "jekyll-gist"
+  # gem "jekyll-feed"
+  # gem "jemoji"
+# end