Document how to create a custom adjustment type

[mglaman]

https://www.drupal.org/project/commerce/issues/3018951

[cmcintosh]

:/ Any updates to this or a file i can look at to get an idea on how adjustments work.

[bojanz]

https://github.com/drupalcommerce/commerce/blob/8.x-2.x/modules/order/commerce_order.commerce_adjustment_types.yml

[lisastreeter]

This topic is covered in this Pricing documentation PR: #382