Added regression tests for navigation

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

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

@@ -0,0 +1,13 @@
+---
+layout: default
+title: Strings
+parent: Order
+nav_order: 2
+has_children: true
+---
+
+# String Order
+
+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).