Update BlockTemplateController.php docs after the refactor

[Aljullu]

In #44537, the render_block_template() method was moved from the BlockTemplateController class into each individual template classes.

However, I didn't update the docs accordingly:

https://github.com/woocommerce/woocommerce/blob/trunk/plugins/woocommerce-blocks/docs/internal-developers/templates/block-template-controller.md#render_block_template

This issue is about updating the docs acknowledging that the function has been moved to the individual template classes.