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/_posts/2012-03-14-layout-excerpt-defined.md

@@ -0,0 +1,15 @@
+---
+title: "Layout: Excerpt (Defined)"
+excerpt: "This is a user-defined post excerpt. It should be displayed in place of the post content in archive-index pages."
+categories:
+  - Layout
+  - Uncategorized
+tags:
+  - content
+  - excerpt
+  - layout
+---
+
+This is the post content. It should be displayed in place of the user-defined excerpt in archive-index pages.
+
+This paragraph should be absent from an archive-index page where `post.excerpt` is shown.