Added regression tests for navigation

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

docs/tests/navigation/order/default/10.md

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+---
+layout: default
+title: "10"
+parent: Default
+grand_parent: Order
+---
+
+# 10
+
+```yaml
+title: "10"
+parent: Default
+grand_parent: Order
+```

docs/tests/navigation/order/default/2.md

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+---
+layout: default
+title: "2"
+parent: Default
+grand_parent: Order
+---
+
+# 2
+
+```yaml
+title: "2"
+parent: Default
+grand_parent: Order
+```

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

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+---
+layout: default
+title: A
+parent: Default
+grand_parent: Order
+---
+
+# A
+
+```yaml
+title: A
+parent: Default
+grand_parent: Order
+```

docs/tests/navigation/order/default/aa-lower.md

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+---
+layout: default
+title: aa
+parent: Default
+grand_parent: Order
+---
+
+# aa
+
+```yaml
+title: aa
+parent: Default
+grand_parent: Order
+```

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

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+---
+layout: default
+title: Aa
+parent: Default
+grand_parent: Order
+---
+
+# Aa
+
+```yaml
+title: Aa
+parent: Default
+grand_parent: Order
+```

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

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+---
+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).