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/_pages/portfolio-archive.html

@@ -0,0 +1,14 @@
+---
+layout: archive
+title: "Portfolio"
+permalink: /portfolio/
+author_profile: false
+---
+
+{% include base_path %}
+
+<div class="grid__wrapper">
+  {% for post in site.portfolio %}
+    {% include archive-single.html type="grid" %}
+  {% endfor %}
+</div>