docs(sdk): add complex tests page

[leandrodamascena]

closes #172

Description of changes:

This PR adds docs/testing-patterns/complex-workflows.md documentation for testing complex workflows in durable functions.

The guide covers:

• Multi-step workflows - Patterns for testing functions with multiple sequential operations
• Nested child contexts - How to test workflows that use child contexts for organization
• Parallel operations - Verifying that concurrent operations execute correctly
• Error scenarios - Testing error handling, retries, and recovery patterns
• Timeout handling - How to test timeout scenarios and long-running operations
• Polling patterns - Testing wait_for_condition and similar polling workflows with examples
• State verification - Advanced patterns for inspecting workflow state at different stages
• Complex assertions - How to verify multiple operations and their interactions

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.