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/2012-03-15-layout-header-image-horizontal.md

@@ -0,0 +1,20 @@
+---
+title: "Layout: Header Image (Horizontal)"
+header:
+  image: /assets/images/unsplash-image-1.jpg
+  caption: "Photo credit: [**Unsplash**](https://unsplash.com)"
+categories:
+  - Layout
+  - Uncategorized
+tags:
+  - edge case
+  - featured image
+  - image
+  - layout
+---
+
+This post should display a **header image**, if the theme supports it.
+
+Non-square images can provide some unique styling issues.
+
+This post tests a horizontal header image.