docs: [#14] add multi-provider infrastructure abstraction plan

- Create comprehensive plan for supporting multiple infrastructure providers
- Document proposed architecture with provider-specific modules
- Include detailed implementation plan with 5-week timeline
- Add Hetzner provider implementation examples
- Maintain twelve-factor app compliance throughout
- Enable easy addition of future providers (AWS, GCP, etc.)
- Preserve backward compatibility with existing local workflow

This plan enables the repository to support multiple cloud providers
while maintaining clean separation between infrastructure provisioning
and application deployment as per twelve-factor methodology.

Author: josecelano