docs: add span events documentation for chat completion traces

[devin-ai-integration]

Add Span Events Documentation for Chat Completion Traces

This PR adds comprehensive documentation for span events in chat completion traces, including:

Changes

• Added span events documentation to POST /api/trace endpoint reference
• Updated tracing guide with token streaming and function call examples
• Added detailed examples for both Python and JavaScript SDKs
• Included visual documentation of trace events in the UI

Documentation Updates

• Added span events section to API reference
• Enhanced OpenAI chat completion examples with streaming
• Added function call event documentation
• Updated attribute documentation for all event types

Testing

• Verified documentation accuracy against implementation
• Confirmed example code functionality
• Validated all links and references
• Checked formatting and structure consistency

Link to Devin run: https://preview.devin.ai/devin/ca53be04abf140798c555b14a2b379d5

[devin-ai-integration]

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