Improve retry logic and add tests for server error handling (#32)

Refines GotenbergClient's retry logic to distinguish between retryable and non-retryable server errors based on status codes and error message patterns. Adds an internal isRetryableServerError method and updates sendRequestWithRetry to use it for 500 errors. Introduces comprehensive unit tests in RetryBehaviorTests.swift to verify correct retry behavior for various error scenarios. Updates README with improved documentation and usage examples.

Author: thoven87