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/analytics.html

@@ -0,0 +1,12 @@
+{% if site.analytics.provider and page.analytics != false %}
+
+{% case site.analytics.provider %}
+{% when "google" %}
+  {% include /analytics-providers/google.html %}
+{% when "google-universal" %}
+  {% include /analytics-providers/google-universal.html %}
+{% when "custom" %}
+  {% include /analytics-providers/custom.html %}
+{% endcase %}
+
+{% endif %}