Update ADK doc according to issue #1438 - 1

[adk-bot]

1. Document new A2A capabilities: Reasoning, Long-Running Tools, and Artifacts

Doc file: docs/a2a/intro.md

Current state:

The introduction covers the "What", "When", and "How" of A2A but does not detail the specific capabilities supported by the protocol implementation, such as reasoning models or artifact streaming.

Proposed Change:

Add a new section "Supported Capabilities" that outlines the advanced features supported by the ADK A2A implementation:

1. Reasoning Models: Support for transmitting "thought" traces from reasoning models (like Gemini 2.0 Flash Thinking).
2. Long-Running Tools: Built-in handling for tools that require significant time to execute.
3. Artifact Streaming: Support for streaming large data or files as artifacts between agents.

Reasoning:
The new converter files (part_converter.py, long_running_functions.py, from_adk_event.py) introduce support for these advanced capabilities. Users building complex agent systems need to know that the A2A protocol in ADK supports these features.

Reference: src/google/adk/a2a/converters/part_converter.py

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