Update tutorial with light mode design and sequential navigation

Implemented three key improvements based on user feedback:

1. **Light Mode Design**: Replaced dark slate backgrounds with clean light
   design (bg-gray-50, white cards, dark text) while maintaining orange
   theme accents for consistency

2. **Sequential Navigation**: Added "Next" buttons after each tutorial step
   that automatically advance to the next tab:
   - Step 1 → "Next: Add Categories & Timestamps"
   - Step 2 → "Next: Build Beautiful News Cards"
   - Step 3 → "Next: View API Reference"

3. **API Reference Tab**: Created dedicated final tab (Tab 4) with complete
   API documentation and "Back to Step 1" button for tutorial loop

The tutorial now provides a clear learning path through 4 progressive steps
with improved readability and user guidance.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: stenhougaard