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/_pages/category-archive.html

@@ -0,0 +1,17 @@
+---
+layout: archive
+permalink: /categories/
+title: "Posts by Category"
+author_profile: true
+---
+
+{% include base_path %}
+{% include group-by-array collection=site.posts field="categories" %}
+
+{% for category in group_names %}
+  {% assign posts = group_items[forloop.index0] %}
+  <h2 id="{{ category | slugify }}" class="archive__subtitle">{{ category }}</h2>
+  {% for post in posts %}
+    {% include archive-single.html %}
+  {% endfor %}
+{% endfor %}