Add deprecation notice and redirect to new Polaris library

[divinejojolola-shopify]

WHY are these changes introduced?

Fixes #979 issue

Polaris React is being deprecated in favour of the new web Polaris library built with web components. Users need to be clearly informed about this deprecation and directed to the new library to ensure a smooth migration.

WHAT is this pull request doing?

This PR updates the global banner in the Polaris React documentation to:

• Mark the library as deprecated

Changes made:

• Updated banner text from "Introducing Polaris - Unified and for the web" to deprecation notice

This ensures developers are aware they should migrate to the new Polaris library

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

🎩 checklist

• Tested a snapshot
• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide

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