Added regression tests for navigation

See the change to `_config.yml` for how to activate the tests.

docs/tests/navigation/order/default/index.md

@@ -0,0 +1,15 @@
+---
+layout: default
+title: Default
+parent: Order
+nav_order: 1
+has_children: true
+---
+
+# Default Order
+
+When `nav_order` fields are omitted, the pages are ordered alphabetically by their titles.
+
+By default, all Capital letters come before all lowercase letters; you can add `nav_sort: case_insensitive` in the configuration file to ignore the case).
+
+Digits precede letters, and numeric titles are ordered lexicographically: `10` precedes `2` (in contrast to explicit numeric `nav_order` values).