docs: add comprehensive useOpenAiGlobal hook guide

- Create complete guide at packages/ui/docs/guides/useOpenAiGlobal.md
  * Overview with data flow diagram
  * Quick start examples (server and widget)
  * Data structure patterns (search, map, product, analytics, weather)
  * Design best practices for structuredContent
  * Full hook API documentation
  * Type-safe usage patterns
  * Common patterns with real-world examples
  * Troubleshooting guide
  * Browser compatibility table
  * Best practices and recommendations
  * Testing & mocking guide
  * Architecture explanation

- Add Guides section to docs index
  * Link to new useOpenAiGlobal guide
  * Link to existing performance guide

- Add documentation link in Storybook story
  * Prominent info box in OpenAiHooks playground
  * Direct link to complete guide with hover effects
  * Points to data structure patterns and best practices

This helps users understand the full capabilities of the hook without
confusion, with focus on correct usage from day one.

Author: JakeLin