Added regression tests for navigation

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

docs/tests/navigation/grandparent/a.md

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+---
+layout: default
+title: A
+has_children: true
+---
+
+# A
+
+A top-level page
+
+```yaml
+title: A
+has_children: true
+```

docs/tests/navigation/grandparent/b.md

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+---
+layout: default
+title: B
+has_children: true
+---
+
+# B
+
+A top-level page
+
+```yaml
+title: B
+has_children: true
+```

docs/tests/navigation/grandparent/ca.md

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+---
+layout: default
+title: C
+parent: A
+has_children: true
+---
+
+# C
+
+A child of page A, and parent of page D
+
+```yaml
+title: C
+parent: A
+has_children: true
+```

docs/tests/navigation/grandparent/cb.md

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+---
+layout: default
+title: C
+parent: B
+has_children: true
+---
+
+# C
+
+A child of page B, and parent of page D
+
+```yaml
+title: C
+parent: B
+has_children: true
+```

docs/tests/navigation/grandparent/dca.md

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+---
+layout: default
+title: D
+parent: C
+grand_parent: A
+---
+
+# D
+
+A grandchild of page A
+
+```yaml
+title: D
+parent: C
+grand_parent: A
+```

docs/tests/navigation/grandparent/dcb.md

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+---
+layout: default
+title: D
+parent: C
+grand_parent: B
+---
+
+# D
+
+A grandchild of page B
+
+```yaml
+title: D
+parent: C
+grand_parent: B
+```