Create mixed-provider example templates

**Summary**: Develop example templates and documentation demonstrating best practices for using multiple IaC providers within a single azd project.

**Acceptance Criteria**:
- [ ] Example project with Bicep + Terraform layers
- [ ] Documentation for mixed-provider patterns and best practices
- [ ] Parameter passing examples between different providers
- [ ] Common scenarios (global resources in one provider, regional in another)
- [ ] Testing and validation examples for mixed-provider setups
- [ ] Migration guides for converting single-provider projects