BRP-17548: Add clarity around redirectUrl and testing

[catherinevalek]

BRP-17548

What changed?

• Creating a checkout - GraphQL Account API: Unified billing - Adds this within the first paragraph of this section: “When the merchant exits the Bigcommerce checkout UI, they will be redirected to the redirectUrl provided in the request. Use this field to route the merchant back to your app.”

• Fetch a checkout - GraphQL Account API: Unified billing - Adds this within the first paragraph of the section: “Once the checkout is created, you can periodically poll the status of checkout by calling this endpoint.”

• Test Unified Billing with your app Unified Billing Overview - Within this section, add the following as a bullet to both app segment sections: “After you create a checkout, you can test completing of the checkout by creating and logging into a sandbox store as the Store Owner, and completing the checkout as a merchant would do. “

• Adds clarification in authentication headers section in Gathering Information. docs/docs/integrations/apps/unified-billing/index.mdx at main · bigcommerce/docs

Release notes draft

Anything else?

ping {names}

[catherinevalek]

We'd also like to add a link to the graphql reference in the Unified Billing sidebar. How would I go about adding that?