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

_data/navigation.yml

@@ -1,12 +1,12 @@
-# main links links
+# main links
 main:
   - title: "Quick-Start Guide"
     url: https://mmistakes.github.io/minimal-mistakes/docs/quick-start-guide/
-  - title: "About"
-    url: https://mmistakes.github.io/minimal-mistakes/about/
-  - title: "Sample Posts"
-    url: /year-archive/
-  - title: "Sample Collections"
-    url: /collection-archive/
-  - title: "Sitemap"
-    url: /sitemap/
+  # - title: "About"
+  #   url: https://mmistakes.github.io/minimal-mistakes/about/
+  # - title: "Sample Posts"
+  #   url: /year-archive/
+  # - title: "Sample Collections"
+  #   url: /collection-archive/
+  # - title: "Sitemap"
+  #   url: /sitemap/