Create excluded.md

Add a test for explicit exclusion of an untitled page.

docs/tests/navigation/exclusion/excluded.md

@@ -0,0 +1,11 @@
+---
+layout: default
+nav_exclude: true
+---
+# Tests for exclusion untitled and excluded
+
+This page does not have a `title`, and it is explicitly excluded from the navigation
+
+```yaml
+nav_exclude: true
+```