docs: expand JSONPlaceholder guide with pagination, filtering, and error handling sections

[derrickscottux-collab]

Summary

Enhanced the GUIDE.md with comprehensive pagination, filtering, and error handling documentation based on systematic API testing.

Changes

• Pagination section: Added three pagination approaches (_limit, _page+_limit, _start+_end) with working examples
• Filtering improvements:
  • Added critical case sensitivity warning (incorrect casing silently returns all results)
  • Documented combining multiple filters (AND logic)
  • Added examples for filter + pagination combinations
• Error handling section: Documented different behaviors for invalid resource IDs (404 + empty object) vs invalid filter values (200 + empty array)
• Nested routes clarification: Explicitly stated that nested routes and query parameters are equivalent approaches

Why These Changes Matter

1. Case sensitivity warning: Prevents silent failures where developers use wrong casing and unknowingly receive all results instead of filtered data
2. Pagination examples: The original guide showed basic filtering but no pagination, which is essential for handling large datasets
3. Error handling: Helps developers write proper error handling logic by understanding the different response patterns
4. Filter combinations: Shows how to build more complex queries, which wasn't previously documented

Testing

All examples were tested using Postman against the live JSONPlaceholder API. Complete test results available upon request.

Maintains Existing Style

• Preserved original structure and formatting
• Used consistent code example format
• Kept existing educational tone and "Important" callouts