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-docs/user-guide/integrations-api/integrations.mdx

@@ -15,7 +15,7 @@ Twenty currently offers native integration with:
 - **Email & Calendar**: Connect Gmail, Outlook, or SMTP/CalDAV providers
 - **API Access**: Use our REST and GraphQL APIs to build custom integrations
 
-For email and calendar setup, visit [Email & Calendar Setup](/user-guide/section/settings/email-calendar-setup).
+For email and calendar setup, visit [Email & Calendar Setup](/user-guide/settings/email-calendar-setup).
 
 ### Workflows (Recommended)
 The primary way to connect Twenty to other tools is through our in-app **Workflows** feature:
@@ -24,7 +24,7 @@ The primary way to connect Twenty to other tools is through our in-app **Workflo
 - **Webhook Triggers**: Receive data from external systems
 - **Field Update Triggers**: Automate actions based on CRM changes
 
-Learn more in our [Workflows section](/user-guide/section/workflows/getting-started-workflows).
+Learn more in our [Workflows section](/user-guide/workflows/getting-started-workflows).
 
 ### Zapier Integration (Legacy)
 We maintain a Zapier integration for users who prefer no-code automation: