Update ADK doc according to issue #1173 - 8 - LiteLLM file input, model support

[adk-bot]

8. Summary of the change

Doc file: docs/agents/models/litellm.md

Current state:

The documentation covers basic usage but doesn't mention file handling capabilities or the warning about using Gemini via LiteLLM.

Proposed Change:

1. Mention that file inputs (images, PDFs, etc.) are automatically handled for supported providers (OpenAI, Azure) by uploading them as files when necessary.
2. Add a note about the warning when using LiteLlm(model="gemini/...") suggesting the native Gemini model class for better performance.

Reasoning:
LiteLLM integration has been updated to support automatic file uploads for OpenAI/Azure, and now warns users when using Gemini models via LiteLLM instead of the native integration. These are important for usability and performance.

Reference: src/google/adk/models/lite_llm.py

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