Create a default Promotion(s)

Problem/Motivation

commerce_demo module does not feature any promotion content entity. commerce_kickstart_demo should come with a promotion or multiple promotions to showcase how promotions in Commerce are working.

Potentially here are some scenarios that we could cover:
- a standalone promotion that activates automatically for products in a specific category/certain variation types.
- a promotion that is activated via coupon code, ex. 10PERCENTOFF
- a promotion that applies on the order item level vs a promotion that applies on the order total level