chore(docs): update TOC

Author: Haruma-K

README.md

@@ -36,6 +36,7 @@ Hierarchical debug menu system for Unity that makes it easy to create intuitive
   - [Workflow in multiple scenes](#workflow-in-multiple-scenes)
   - [Exclude from the release builds](#exclude-from-the-release-builds)
   - [Custom Cells](#custom-cells)
+  - [Extending Built-in Cell Prefabs](#extending-built-in-cell-prefabs)
 - [Advanced Usage](#advanced-usage)
   - [Use async methods instead of coroutines](#use-async-methods-instead-of-coroutines)
   - [Hide Backdrop](#hide-backdrop)

README_JA.md

@@ -46,6 +46,7 @@
   - [最小・最大サイズの調整](#%E6%9C%80%E5%B0%8F%E3%83%BB%E6%9C%80%E5%A4%A7%E3%82%B5%E3%82%A4%E3%82%BA%E3%81%AE%E8%AA%BF%E6%95%B4)
   - [デザインをカスタムする](#%E3%83%87%E3%82%B6%E3%82%A4%E3%83%B3%E3%82%92%E3%82%AB%E3%82%B9%E3%82%BF%E3%83%A0%E3%81%99%E3%82%8B)
   - [まとめて戻る](#%E3%81%BE%E3%81%A8%E3%82%81%E3%81%A6%E6%88%BB%E3%82%8B)
+  - [組み込みセルPrefabを拡張する](#%E7%B5%84%E3%81%BF%E8%BE%BC%E3%81%BF%E3%82%BB%E3%83%ABprefab%E3%82%92%E6%8B%A1%E5%BC%B5%E3%81%99%E3%82%8B)
 - [拡張パッケージ](#%E6%8B%A1%E5%BC%B5%E3%83%91%E3%83%83%E3%82%B1%E3%83%BC%E3%82%B8)
   - [Unityのシステム情報を表示する](#unity%E3%81%AE%E3%82%B7%E3%82%B9%E3%83%86%E3%83%A0%E6%83%85%E5%A0%B1%E3%82%92%E8%A1%A8%E7%A4%BA%E3%81%99%E3%82%8B)
   - [In-game Debug Console](#in-game-debug-console)