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/_posts/2010-08-07-post-image-caption.md

@@ -0,0 +1,19 @@
+---
+title: "Post: Image (Caption)"
+categories:
+  - Post Formats
+tags:
+  - image
+  - Post Formats
+---
+
+{% include base_path %}
+
+{% capture fig_img %}
+![Foo]({{ basepath }}/assets/images/unsplash-gallery-image-3.jpg)
+{% endcapture %}
+
+<figure>
+  {{ fig_img | markdownify | remove: "<p>" | remove: "</p>" }}
+  <figcaption>Photo from Unsplash.</figcaption>
+</figure>