add toc-tree demo section

exampleSite/content/toc-tree/_index.md

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+---
+title: ToC Tree
+geekdocFlatSection: true
+---
+
+This is just a demo section for the [toc-tree](/shortcodes/toc-tree/) shortcode.
+
+{{< toc-tree >}}

exampleSite/content/toc-tree/level-1/_index.md

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+Level 1
+
+{{< toc-tree >}}

exampleSite/content/toc-tree/level-1/level-1-1.md

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+---
+title: Level 1.1
+---
+
+Level 1.1

exampleSite/content/toc-tree/level-1/level-1-2.md

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+---
+title: Level 1.2
+---
+
+Level 1.2

exampleSite/content/toc-tree/level-1/level-1-3/_index.md

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+---
+title: Level 1.3
+---
+
+Level 1.3
+
+{{< toc-tree >}}

exampleSite/content/toc-tree/level-1/level-1-3/level-1-3-1.md

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+---
+title: Level 1.3.1
+---
+
+Level 1.3.1

exampleSite/content/toc-tree/level-2/_index.md

@@ -0,0 +1 @@
+Level-2

exampleSite/content/toc-tree/level-2/level-2-1.md

@@ -0,0 +1,5 @@
+---
+title: Level 2.1
+---
+
+Level 2.1

exampleSite/content/toc-tree/level-2/level-2-2.md

@@ -0,0 +1,5 @@
+---
+title: Level 2.2
+---
+
+Level 2.2