docs: document using Smooth Generator in PHPUnit tests

[faisalahammad]

All Submissions:

• Have you followed the Contributing guidelines?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Adds documentation on using Smooth Generator inside PHPUnit tests, addressing issue #151. The generators are already callable as static methods (Product::generate(), Product::batch(), Router::generate_batch()) and the plugin's own test suite uses them this way, but there was no guidance for extension developers who want the same in their own suites.

New file docs/using-in-phpunit.md covers: prerequisites (WP + WooCommerce bootstrapped, plugin loaded), a quick start, per-generator code examples (products, orders, customers, coupons, bookings, terms, router), return-value and assertion patterns, the smoothgenerator_*_generated action hooks, and gotchas (batch counts can be shorter than requested, Term's positional taxonomy argument, orders need products, bookings need WC Bookings, no image-disable flag, 100 batch cap, emails auto-disabled). The README links to it.

Closes #151.

How to test the changes in this Pull Request:

1. Read docs/using-in-phpunit.md.
2. Confirm every code snippet matches the generator signatures in includes/Generator/*.php and includes/Router.php.
3. Optionally copy a snippet into a WP_UnitTestCase that boots WooCommerce and run it.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable? (Docs only; no code changed, so no new tests.)
• Have you successfully run tests with your changes locally? (No functional change; composer phpcs passes for changed content.)

Changelog entry

Add documentation for using Smooth Generator inside PHPUnit tests.

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.