feat: Add migration guide for PostgreSQL from Bitnami to CloudPirates

[mgarciaLKS]

Add Generic PostgreSQL Migration Guide (Bitnami → CloudPirates)

This PR introduces a comprehensive migration guide for transitioning PostgreSQL deployments from Bitnami Helm charts to CloudPirates Helm charts in Kubernetes environments.

What's Added

• File: migration-guides/GENERIC_POSTGRESQL_MIGRATION_GUIDE.md
• Purpose: Reusable migration procedure with step-by-step instructions
• Scope: Generic guide adaptable to any project using these chart types

Key Features

• ✅ Complete backup and restore workflow using pg_dumpall
• ✅ Chart configuration updates (Chart.yaml and values.yaml examples)
• ✅ Data integrity verification steps
• ✅ Rollback procedures for safe migration
• ✅ Troubleshooting section for common issues
• ✅ Supports PostgreSQL 12.x/15.x → 18.x migration