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/_sass/vendor/susy/_su.scss

@@ -0,0 +1,4 @@
+// Su
+// ==
+
+@import 'susy/su';