Added tutorial Modeling assortments with Product Selections

In the new tutorial Modeling assortments with Product Selections, users of Composable Commerce will learn:

• How to use Product Selections and Stores to model assortments that you want to make available on your storefronts.
• How to use Store-specific API clients to access only the products available in that Store.
• How to include specific Product Variants in an assortment.
• How to temporarily exclude entire Products from the assortment.
• How to retrieve the Products in a Store using the Product Projection API and the Product Search API.