Create a Promotions page for Developers

[joshmiller83]

1. Create a good introduction that describes the promotions and how they are built from a technical perspective (what are plugins, what events do they use, etc). https://docs.drupalcommerce.org/commerce2/developer-guide/promotions
2. Add information on the exclusive condition that comes with promotions (link to the core component page for more information) https://docs.drupalcommerce.org/commerce2/developer-guide/promotions/create-a-condition
   • https://github.com/drupalcommerce/commerce/blob/8.x-2.x/modules/promotion/src/Plugin/Commerce/Condition/OrderItemQuantity.php

[lisastreeter]

Good ideas. I started working on a "Pricing" section for the documentation, to cover prices, currencies, promotions (and fees, once that submodule lands). I have an almost-complete page for Offer plugins but when I started working on a similar page for Promotion Conditions, I realized that they were really a "Core component" rather than just a component of "Promotions". So I switched gears temporarily to work on a Core conditions documentation page: #270

And I'm thinking I should do the same for "Resolvers".