docs: Add OpenTelemetry instrumentation guide for Copilot SDK (#529)

Add comprehensive documentation for instrumenting Copilot SDK applications
with OpenTelemetry GenAI semantic conventions.

The guide includes:
- Installation and basic setup for OpenTelemetry
- Complete SessionEventType to GenAI attribute mapping
- Detailed examples for ASSISTANT_USAGE, TOOL_EXECUTION events
- Full working code example with proper span management
- Content recording guidelines (opt-in for sensitive data)
- MCP tool conventions
- Metrics export
- Azure Monitor integration
- Best practices and troubleshooting

This enables Copilot SDK users to add standardized observability
following OpenTelemetry GenAI Semantic Conventions v1.34.0.

Author: tonybaloney