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/_includes/base_path

@@ -0,0 +1,5 @@
+{% if site.url %}
+  {% assign base_path = site.url | append: site.baseurl %}
+{% else %}
+  {% assign base_path = site.github.url %}
+{% endif %}