docs: add termination condition best practices guide

[lawcontinue]

Addresses #5870

Summary

Adds a best practices guide for choosing termination conditions in AgentChat.

The existing termination tutorial covers the API, but doesn't guide users on when to use which condition. This is especially important because TextMentionTermination can be unreliable with tool-using agents — the model may never output the expected keyword in plain text.

What's added

• New file: termination-best-practices.md in the agentchat user guide
  • When TextMentionTermination works (and when it doesn't)
  • Recommended pattern: always combine with deterministic conditions
  • Deterministic alternatives: MaxMessageTermination, SourceMatchTermination, HandoffTermination, ExternalTermination, TimeoutTermination, TokenUsageTermination
  • Stop-and-resume pattern for interactive web UIs
• Updated: index.md to link the new guide

Testing

• Built locally with sphinx — renders correctly
• All code examples verified against autogen-agentchat 0.4 API

Related

• Update documentation to replace usage of TextMentionTermination that relies on models in favor of more robust conditions #5870
• Tool usage and spurious messages spilling in RoundRobinGroupChat #5836