docs: Add AG2 framework integration page

[VasiliyRad]

Summary

• Add new integration page for AG2 (pages/integrations/frameworks/ag2.mdx)

Why

AG2 is an open-source multi-agent framework (4k+ GitHub stars) that ships built-in OpenTelemetry tracing since v0.11. This tracing emits rich hierarchical spans (conversations, agent turns, LLM calls, tool executions, speaker selection) following GenAI semantic conventions — all of which Langfuse can ingest natively via its OTel endpoint.

The existing autogen.mdx page documents integration via OpenLIT as an intermediary, which only captures top-level spans and misses per-call LLM details. AG2 is a separate project from Microsoft's AutoGen (which has stopped active development), so this is a new page rather than a replacement.

The AG2 page shows direct OTel export to Langfuse — no intermediary library needed — giving users full trace depth including token usage, costs, tool arguments/results, and group chat speaker selection.

What's in the page

• Step-by-step setup: install, configure Langfuse credentials, set up OTel exporter, instrument agents
• Span type mapping table (AG2 span types → Langfuse observations)
• Tool use example
• Group chat example with instrument_pattern
• A2A distributed tracing
• Environment variable alternative setup
• Migration note for users coming from the OpenLIT-based approach

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