Update ADK doc according to issue #1438 - 15

[adk-bot]

15. Document render_ui_widgets in EventActions

Doc file: docs/events/index.md

Current state:

• Control Flow Signals: Check boolean flags or string values:
  * event.actions.transfer_to_agent (string): Control should pass to the named agent.
  * event.actions.escalate (bool): A loop should terminate.
  * event.actions.skip_summarization (bool): A tool result should not be summarized by the LLM.

Proposed Change:

• Control Flow Signals & UI:
  * event.actions.transfer_to_agent (string): Control should pass to the named agent.
  * event.actions.escalate (bool): A loop should terminate.
  * event.actions.skip_summarization (bool): A tool result should not be summarized by the LLM.
  * event.actions.render_ui_widgets (list[UiWidget]): A list of UI widgets to be rendered by the client.

Reasoning:
The render_ui_widget method and UiWidget support were added to allow agents to send UI components. This is a new capability that should be documented in the Events section.

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

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