Allow creating documents without order reference

[shyim]

Description
Currently Shopware\Core\Checkout\Document\DocumentGenerator\DocumentGeneratorInterface::generate() expects to be passed an OrderEntity.

It should be possible though to create documents unrelated to orders without having to copy Shopware's own classes into a plugin and remove all order references.

Possible Solution
For order documents the order could be integrated into a special kind of DocumentConfiguration (e.g. OrderDocumentConfiguration), while DocumentConfiguration would be turned into an abstract class (or interface) to allow different kind of configurations.