Update ADK doc according to issue #1438 - 16

[adk-bot]

16. Document render_ui_widget method in Context documentation

Doc file: docs/context/index.md

Current state:

• State for Data Flow: context.state is the primary way to share data...
• Artifacts for Files: Use context.save_artifact and context.load_artifact...
• Tracked Changes: Modifications to state or artifacts...

Proposed Change:

• State for Data Flow: context.state is the primary way to share data...
• Artifacts for Files: Use context.save_artifact and context.load_artifact...
• UI Rendering: Use context.render_ui_widget(widget) to send rich UI components (like MCP App iframes) to the client.
• Tracked Changes: Modifications to state or artifacts...

Reasoning:
The render_ui_widget method was added to the Context class. Documenting it in the "Common Tasks" or "Key Takeaways" section of the Context docs ensures developers know how to use this new feature.

Reference: src/google/adk/agents/context.py

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