Docs: Add a tutorial for the checkout workflow

[keharper]

Create a GraphQL tutorial for the checkout workflow. Conceptually, it should be similar to REST tutorials, such as Order Processing.

Primary source: https://github.com/TomashKhamlai/graphql-ce/wiki/Checkout-workflow

This issue replaces devdocs issue magento/devdocs#3715

[atwixfirster]

PR describes basic flow: magento/devdocs#4271