feat: Add Migrating from Heroku to DigitalOcean guide

[bobbyonmagic]

Comprehensive migration guide covering three paths from Heroku to DigitalOcean.

What This Adds

Three Migration Paths:

1. App Platform - Managed PaaS (60-80% cost savings vs Heroku)
2. Coolify Self-Hosted - Open-source alternative on Droplets (90%+ savings)
3. Hybrid Approach - Coolify for staging, App Platform for production

Content Coverage

• Complete cost comparisons with real numbers
• App Platform setup (git deployments, auto-scaling, zero-downtime)
• Managed Database migration (PostgreSQL with logical replication)
• Redis, Spaces (S3-compatible), and managed services
• Coolify installation and deployment walkthrough
• Database backup strategies and best practices
• Infrastructure-as-code examples (Terraform)
• Real-world migration timelines and strategies

Key Features

• Natural, narrative writing style (not robotic bullet points)
• DigitalOcean affiliate links throughout
• Cost comparison tables showing 94-97% savings with Coolify
• Production-ready migration steps with minimal downtime
• Decision framework for choosing the right path

Testing

• ✅ All 4,641 tests passing
• ✅ OG images generated
• ✅ Markdown validation passing
• ✅ 15-minute read, ~3,000 words