Add documentation page to llm-canvas webUI

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This PR implements a comprehensive documentation page for the llm-canvas web interface, making existing documentation easily accessible to users through the UI.

🆕 Features Added

Documentation Page Component

• New route: /docs accessible from anywhere in the app
• Responsive sidebar navigation with 4 main documentation sections:
  1. Getting Started - Installation, server setup, and basic usage examples
  2. Canvas Management - Git-inspired workflow concepts, branching, and merging
  3. Message Types - Rich content support and block types
  4. Server Setup - Local server configuration and options

Enhanced Navigation

• Added "Docs" button with Book icon to the main header for easy access
• Mobile-responsive design with collapsible sidebar and hamburger menu
• Consistent styling that matches existing UI patterns

🔧 Technical Implementation

• Zero dependencies added - Uses existing MarkdownRenderer component for consistent display
• Follows established patterns - React Router integration, Tailwind CSS styling
• Mobile-first responsive design - Works seamlessly on desktop and mobile devices
• Syntax highlighting - Code examples with copy buttons using existing infrastructure
• Accessible - Proper ARIA labels and keyboard navigation support

📱 Screenshots

Desktop View:

Mobile View:

🎯 Benefits

• Makes LLM Canvas self-documenting and more user-friendly
• Reduces barrier to entry for new users
• Provides comprehensive guidance on canvas management, message types, and server setup
• Maintains consistency with existing UI patterns and accessibility standards

The documentation content is embedded directly in the component to ensure it's always available and doesn't require external file loading, making it reliable for both local and GitHub Pages deployment modes.

Fixes #21.

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