[ContentUnderstanding] Add preview disclaimer for to_llm_input

[chienyuanchang]

Description

Adds a preview feature disclaimer to the to_llm_input() section in the README using GitHub admonition syntax (> [!NOTE]), directing users to file feedback on GitHub Issues with the Cognitive - Content Understanding label.

Cross-language change requested in Azure/azure-sdk-for-java#49000.

Changes

• README.md: Added > [!NOTE] preview disclaimer above the to_llm_input() usage section

Related PRs

• Java: [ContentUnderstanding] Add LlmInputHelper for converting analysis results to LLM-friendly text azure-sdk-for-java#49000
• .NET: [ContentUnderstanding] Add preview disclaimer for ToLlmInput azure-sdk-for-net#58929
• JS: https://github.com/Azure/azure-sdk-for-js/pull/new/cu_sdk/llm_input_preview_note

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message.

Testing Guidelines

• Pull request includes test coverage for the included changes.
  • Documentation-only change; no code affected.

[Copilot]

Pull request overview

This PR updates the azure-ai-contentunderstanding package README to add an explicit preview disclaimer for the to_llm_input() helper, directing users to provide feedback via GitHub Issues.

Changes:

• Added a preview feature disclaimer note above the to_llm_input() usage section in the README.