Update SUMMARY.md

This has been designed by @Jowan-Spooner

documentation/SUMMARY.md

@@ -1,21 +1,28 @@
-# Summary
-
 [Introduction](introduction.md)
-[Getting Started](getting-started.md)
+    - [Getting Started](getting-started.md)
 [FAQ](faq.md)
 
+
 - [Character & Portraits](characters-and-portraits.md)
+    - [Custom Portraits](custom-portraits.md)
+
 - [Styles & Layouts](styles-and-layouts.md)
+    - [Dialogic Nodes](dialogic-nodes.md)
+
 - [Variables](variables.md)
-- [Timeline Text Syntax](timeline-text-syntax.md)
-- [Dialogic Nodes](dialogic-nodes.md)
-- [Dialogic Signals](dialogic-signals.md)
-- [Custom Portraits](custom-portraits.md)
-- [Creating Extensions](creating-extensions.md)
-- [Creating Timelines In Code](creating-timelines-in-code.md)
-- [Auto-Advance](auto-advance.md)
-- [Auto-Skip](auto-skip.md)
-- [Translations](translation.md)
-- [Reference Manager](reference-manager.md)
-- [Pausing](pausing.md)
-- [Saving & Loading](saving-and-loading.md)
+
+- Timelines
+    - [Timeline Text Syntax](timeline-text-syntax.md)
+    - [Creating Timelines In Code](creating-timelines-in-code.md)
+
+- Input
+    - [Auto-Advance](auto-advance.md)
+    - [Auto-Skip](auto-skip.md)
+
+- Utilities
+    - [Dialogic Signals](dialogic-signals.md)
+    - [Pausing](pausing.md)
+    - [Saving & Loading](saving-and-loading.md)
+    - [Reference Manager](reference-manager.md)
+    - [Creating Extensions](creating-extensions.md)
+    - [Translations](translation.md)