Add deprecation notice for AutoGrid plugin - Use Figma's native tools instead

[Copilot]

This PR adds a prominent deprecation notice to the AutoGrid Figma plugin, informing users that the plugin is deprecated and they should use Figma's native Auto Layout and Grid features instead.

Changes Made

Plugin UI Updates

• Added a prominent red deprecation banner at the top of the plugin interface
• The banner includes a clear "⚠️ DEPRECATED" warning with guidance to use Figma's native tools
• Styled with high contrast (red background, white text) for maximum visibility
• Positioned at the top of the UI without breaking existing functionality

Documentation Updates

• Updated README.md with a comprehensive deprecation notice at the top
• Clearly states the plugin is no longer maintained
• Provides specific alternatives:
  • Auto Layout: For automatic spacing and alignment
  • Grid Layout: For grid-based designs
  • Layout Grids: For design alignment
• Includes link to Figma Help Center for more information
• Preserves original documentation under "Legacy Documentation" section

Why This Change?

Figma now provides native Auto Layout and Grid features that offer better functionality and performance than this plugin. This deprecation notice helps users transition to the officially supported tools while maintaining access to the plugin during their migration period.

The changes are minimal and surgical - only adding the deprecation notices without modifying any existing plugin logic or functionality.

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