Migrate documentation to Mintlify and configure 301 redirects (#15502)

## Summary
Completes the migration of all documentation from twenty-website to a
new Mintlify-powered documentation site at docs.twenty.com.

## Changes Made

### New Package: `twenty-docs`
- ✅ Created new Mintlify documentation package
- ✅ Migrated 95 content pages (user-guide, developers, twenty-ui)
- ✅ Migrated 81 images
- ✅ Converted all custom components to Mintlify native components
- ✅ Configured navigation with 2 tabs and 94 pages
- ✅ Added Helper AI Agent with searchArticles tool for docs search

### Updated: `twenty-website`
- ✅ Added 11 redirect rules (301 permanent) in next.config.js
- ✅ Removed all documentation content (111 files)
- ✅ Removed documentation routes (user-guide, developers, twenty-ui)
- ✅ Removed documentation components (9 files)
- ✅ Updated keystatic.config.ts
- ✅ Preserved all marketing/release pages

### Updated: Core Files
- ✅ Updated README.md - docs links point to docs.twenty.com
- ✅ Updated CONTRIBUTING.md - code quality link updated
- ✅ Updated SupportDropdown.tsx - user guide link updated
- ✅ Updated Footer.tsx - user guide link updated

packages/twenty-front/src/modules/support/components/SupportDropdown.tsx

@@ -18,7 +18,7 @@ export const SupportDropdown = () => {
   };
 
   const handleUserGuide = () => {
-    window.open('https://twenty.com/user-guide', '_blank');
+    window.open('https://docs.twenty.com/user-guide/introduction', '_blank');
     closeDropdown(dropdownId);
   };