Update ADK doc according to issue #1173 - 2 - DebugLoggingPlugin

[adk-bot]

2. Summary of the change

Doc file: docs/observability/debug-logging.md

Current state:

No documentation for the new DebugLoggingPlugin.

Proposed Change:

Create a new documentation page for DebugLoggingPlugin.

Structure:

1. Overview: Explains that DebugLoggingPlugin captures detailed interaction data (LLM requests/responses, tool calls, events, session state) to a YAML file for debugging.
2. Usage:

   from google.adk.plugins import DebugLoggingPlugin
   
   debug_plugin = DebugLoggingPlugin(output_path="debug_logs.yaml")
   runner = Runner(agent=my_agent, plugins=[debug_plugin])

3. Output Format: Describe the YAML structure (per-invocation documents).
4. Configuration: Document parameters output_path, include_session_state, include_system_instruction.

Reasoning:
New DebugLoggingPlugin is a significant feature for observability and debugging that requires dedicated documentation.

Reference: src/google/adk/plugins/debug_logging_plugin.py

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