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

test/Gemfile

@@ -0,0 +1,6 @@
+source "https://rubygems.org"
+
+# use local theme gem for testing
+gem "minimal-mistakes-jekyll", path: "../"
+
+gem "wdm", "~> 0.1.0" if Gem.win_platform?