docs: Update guides for Quill Editor and ActiveAdmin 4.0.0.beta20

- Replace Tom Select references with Quill Editor
- Update all version references to beta20
- Remove irrelevant guides (Tom Select migration, CKEditor upload system)
- Add Quill Editor specific imports and configurations
- Update package.json dependencies to use Quill instead of Tom Select

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/setup-activeadmin-gem.md

@@ -1,6 +1,6 @@
 # Setting Up an ActiveAdmin 4 Extension Gem (Combustion)
 
-This guide mirrors the working setup used in this repo: ActiveAdmin 4.0.0.beta20 + Tailwind CSS v4 + Combustion test app.
+This guide mirrors the working setup used in this repo: ActiveAdmin 4.0.0.beta20 + Tailwind CSS v4 + test app.
 
 ## 1. Gem Dependencies
 
@@ -48,7 +48,7 @@ end
 
 require "importmap-rails"
 require "active_admin"
-require "activeadmin_your_feature"
+require "activeadmin_quill_editor"
 
 run Combustion::Application
 ```
@@ -78,8 +78,9 @@ end
 @import "tailwindcss";
 @config "../../../tailwind-active_admin.config.mjs";
 
-/* Import your gem CSS if needed */
-@import "your_gem/css";
+/* Import Quill Editor CSS */
+@import "activeadmin/quill_editor/quill.snow";
+@import "activeadmin/quill_editor_input";
 ```
 
 ```javascript
@@ -96,8 +97,8 @@ export default {
     `${activeAdminPath}/vendor/javascript/flowbite.js`,
     `${activeAdminPath}/plugin.js`,
     `${activeAdminPath}/app/views/**/*.{arb,erb,html,rb}`,
-    "./app/admin/**/*.{arb,erb,html,rb}",
-    "./app/views/**/*.{arb,erb,html,rb}",
+    "./app/admin/**/*.{arb,erb,html,rb}`,
+    "./app/views/**/*.{arb,erb,html,rb}`,
     "./app/javascript/**/*.js",
     "../../app/assets/**/*.{js,css}"
   ],
@@ -136,7 +137,11 @@ bundle binstubs tailwindcss-ruby --force
 ```javascript
 // spec/internal/app/javascript/active_admin.js
 import "@activeadmin/activeadmin";
-import "your_gem";
+
+import Quill from "quill";
+window.Quill = Quill;
+
+import "activeadmin_quill_editor";
 ```
 
 ## 10. package.json (internal)
@@ -152,6 +157,7 @@ import "your_gem";
   },
   "devDependencies": {
     "@activeadmin/activeadmin": "^4.0.0-beta20",
+    "quill": "^2.0.3",
     "esbuild": "^0.24.2"
   }
 }