Added documentation for querying promotional banners using the GraphQL Storefront API, including example queries and responses.

DEVDOCS-6492

What changed?

• Storefront GraphQL now supports the querying of promotion banners.

Release notes draft

• We've added support for promotion banners to Storefront GraphQL.

Anything else?

A second pull request in bigcommerce/developer-center will be required.

ping { @bigcommerce/dev-docs-team }