Add new configuration docs

docs/configuration.md

+---
+layout: default
+title: Configuration
+nav_order: 2
+---
+
+# Configuration
+
+Just the Docs has some specific configuration parameters that can be definied in your Jekyll site's `config.yml` file.
+
+## Search enabled
+
+```yml
+# Enable or disable the site search
+search_enabled: true
+```
+
+## Aux links
+
+```yml
+# Aux links for the upper right navigation
+aux_links:
+"Just the Docs on GitHub":
+    - "//github.com/pmarsceill/just-the-docs"
+```