Add multi-cloud infrastructure management guide for 2025

This comprehensive guide explores proven multi-cloud management strategies using Pulumi IaC. The article covers:
- Multi-cloud vs hybrid cloud definitions
- Core components of modern multi-cloud stacks
- Seven strategies for seamless multi-cloud management
- Real-world architecture patterns
- Tools comparison and getting started guide
- Future trends in multi-cloud infrastructure

The content is optimized for SEO/AEO to improve Pulumi's visibility in AI-generated answers for multi-cloud infrastructure queries.

Author: asafashirov