docs: skeleton (#698)

# Summary

- provided bare docs structure
- added some babel configuration for `t-rex-ui` theme
- modified the structure so top-level sections on the sidebar are
interactible, eg `Fundamentals`

Author: hejsztynx

docs/docs/api-reference/_category_.json

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+{
+  "label": "API reference",
+  "position": 50,
+  "link": {
+    "type": "generated-index"
+  }
+}

docs/docs/api-reference/enriched-text-input.md

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+---
+sidebar_position: 1
+---
+
+# EnrichedTextInput
+
+<!-- TODO: write content for this page -->

docs/docs/api-reference/enriched-text.md

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+---
+sidebar_position: 2
+---
+
+# EnrichedText
+
+<!-- TODO: write content for this page -->

docs/docs/core-functionalities/_category_.json

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+{
+  "label": "Core functionalities",
+  "position": 30,
+  "link": {
+    "type": "generated-index"
+  }
+}

docs/docs/core-functionalities/handling-events.md

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+---
+sidebar_position: 3
+---
+
+# Handling events
+
+<!-- TODO: write content for this page -->

docs/docs/core-functionalities/rendering-rich-text.md

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+---
+sidebar_position: 2
+---
+
+# Rendering rich text
+
+<!-- TODO: write content for this page -->

docs/docs/core-functionalities/styling-the-input.md

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+---
+sidebar_position: 1
+---
+
+# Styling the input
+
+<!-- TODO: write content for this page -->

docs/docs/core-functionalities/web-support.md

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+---
+sidebar_position: 4
+---
+
+# Web support
+
+<!-- TODO: write content for this page -->

docs/docs/fundamentals/_category_.json

@@ -0,0 +1,7 @@
+{
+  "label": "Fundamentals",
+  "position": 10,
+  "link": {
+    "type": "generated-index"
+  }
+}

docs/docs/fundamentals/core-concepts.md

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+---
+sidebar_position: 3
+---
+
+# Core concepts
+
+<!-- TODO: write content for this page -->