docs: Fix incorrect API usage examples to use Middleware

Fixed all examples that incorrectly used custom `fetch` function parameter:
- Authentication: Now uses api.use() with onRequest middleware for JWT token injection
- Token Refresh Flow: Uses onRequest and onResponse middleware for automatic token refresh
- Request Cancellation: Uses signal parameter in api.get() options
- Timeout Configuration: Uses middleware with AbortController
- Retry Logic: Uses onResponse middleware for retry on server errors
- Logging & Debugging: Uses onRequest/onResponse middleware
- Caching Strategy: Uses onRequest middleware to return cached responses
- Rate Limiting: Uses onRequest middleware with rate limiter

Removed duplicate "Request Interceptors" section (functionality covered by Middleware examples)

All examples now correctly demonstrate devup-api's Middleware API pattern as shown in test files.

Author: claude