[docs] Scaffold reference guides + README Documentation section

- Documentation/{Extensions,SwiftUI,UIKit}.md shells with category headings + TOC
- README: Documentation table linking all 4 guides
- footer: drop the second github badge, keep the resume link

Documentation/Extensions.md

@@ -0,0 +1,52 @@
+# LCEssentials — Extensions
+
+Foundation, value-type, string, collection, numeric, date, and crypto helpers, plus
+the `LCEssentials` namespace itself. UIKit extensions live in
+[UIKit.md](UIKit.md); SwiftUI helpers in [SwiftUI.md](SwiftUI.md).
+
+Every section is a collapsible block — click a heading to expand it.
+
+## Contents
+
+- [API & Networking](#api--networking)
+- [Strings & Text](#strings--text)
+- [Collections & Sequences](#collections--sequences)
+- [Numbers & Geometry](#numbers--geometry)
+- [Date, Data & Files](#date-data--files)
+- [Encoding & Errors](#encoding--errors)
+- [Crypto](#crypto)
+- [Core — the `LCEssentials` namespace](#core--the-lcessentials-namespace)
+
+---
+
+## API & Networking
+
+<!-- batch 2 -->
+
+## Strings & Text
+
+<!-- batch 3 -->
+
+## Collections & Sequences
+
+<!-- batch 4 -->
+
+## Numbers & Geometry
+
+<!-- batch 5 -->
+
+## Date, Data & Files
+
+<!-- batch 6 -->
+
+## Encoding & Errors
+
+<!-- batch 7 -->
+
+## Crypto
+
+<!-- batch 2 -->
+
+## Core — the `LCEssentials` namespace
+
+<!-- batch 7 -->