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

.github/CONTRIBUTING.md

@@ -1,21 +1,21 @@
 # Contributing to Twenty
 
-Thanks for considering contributing to Twenty! 
+Thanks for considering contributing to Twenty!
 
-Please make sure to go through the [documentation](https://docs.twenty.com) before. 
+Please make sure to go through the [documentation](https://docs.twenty.com) before.
 
 <br>
 
 
 ## Good first issues
 
-Good first issues are a great way to start contributing and get familiar with the codebase. You can find them on by filtering on the [good first issue](https://github.com/twentyhq/twenty/labels/good%20first%20issue) label. 
+Good first issues are a great way to start contributing and get familiar with the codebase. You can find them on by filtering on the [good first issue](https://github.com/twentyhq/twenty/labels/good%20first%20issue) label.
 
 ## Issue assignment
 
 To avoid conflicts, we follow these guidelines:
 
-1. For `Good First Issue` and `Experienced Contributor` issues without `size: long` labels, we'll merge the first PRs that meet our [code quality standards](https://twenty.com/developers). **We don't assign contributors to these issues**. For `priority: high` issues, our core team will step in within days if no adequate contributions are received.
+1. For `Good First Issue` and `Experienced Contributor` issues without `size: long` labels, we'll merge the first PRs that meet our [code quality standards](https://docs.twenty.com/developers). **We don't assign contributors to these issues**. For `priority: high` issues, our core team will step in within days if no adequate contributions are received.
 2. For `size: long` Issues, assigned contributors have one week to submit their first draft PR.
 
 ## How to Contribute