Added regression tests for navigation

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

docs/tests/navigation/order/integers/-1.md

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+---
+layout: default
+title: "-1"
+nav_order: -1
+parent: Integers
+grand_parent: Order
+---
+
+# -1
+
+```yaml
+title: "-1"
+nav_order: -1
+parent: Integers
+grand_parent: Order
+```

docs/tests/navigation/order/integers/0.md

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+---
+layout: default
+title: "0"
+nav_order: 0
+parent: Integers
+grand_parent: Order
+---
+
+# 0
+
+```yaml
+title: "0"
+nav_order: 0
+parent: Integers
+grand_parent: Order
+```

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

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

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

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

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

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+---
+layout: default
+title: Integers
+parent: Order
+nav_order: 3
+has_children: true
+---
+
+# Integer Order
+
+When `nav_order` fields are integers, the pages are ordered in increasing order of the numerical values.
+
+Integers include `0` and negative values. Integers can be reused for top-level pages and for different sets of child pages.