docs: Add OpenTelemetry GenAI semantic conventions setup guide

Add comprehensive documentation for setting up OpenTelemetry tracing with
GenAI semantic conventions based on Azure SDK for Python implementation.

The guide includes:
- Installation instructions for various backends
- Basic setup with code examples
- Complete semantic conventions mapping (v1.34.0)
- Span attributes, events, and metrics reference
- Content recording configuration
- Custom function tracing
- Troubleshooting tips

This documentation is derived from the production implementation in
azure-sdk-for-python and follows the official OpenTelemetry GenAI
Semantic Conventions specification.

Author: tonybaloney