Add product alerts schema

[keharper]

Purpose of this pull request

This PR adds GraphQL documentation for the Product Alerts feature in Adobe Commerce as a Cloud Service (SaaS). It documents two new queries and six new mutations that allow logged-in customers to subscribe to and manage stock availability and price drop alerts for products.

Affected pages

New pages:

• https://developer.adobe.com/commerce/webapi/graphql/schema/products/queries/is-subscribed-product-alert-price/
• https://developer.adobe.com/commerce/webapi/graphql/schema/products/queries/is-subscribed-product-alert-stock/
• https://developer.adobe.com/commerce/webapi/graphql/schema/products/mutations/subscribe-product-alert-price/
• https://developer.adobe.com/commerce/webapi/graphql/schema/products/mutations/subscribe-product-alert-stock/
• https://developer.adobe.com/commerce/webapi/graphql/schema/products/mutations/unsubscribe-product-alert-price/
• https://developer.adobe.com/commerce/webapi/graphql/schema/products/mutations/unsubscribe-product-alert-price-all/
• https://developer.adobe.com/commerce/webapi/graphql/schema/products/mutations/unsubscribe-product-alert-stock/
• https://developer.adobe.com/commerce/webapi/graphql/schema/products/mutations/unsubscribe-product-alert-stock-all/
  Updated pages:
• https://developer.adobe.com/commerce/webapi/graphql/schema/products/queries/
• https://developer.adobe.com/commerce/webapi/graphql/schema/products/mutations/

Links to Magento Open Source code

• https://github.com/magento/commerce-core-saas-service/tree/main/application/modules/Magento/ProductAlertGraphQl

Additional information

• Add new-topic label

What's New highlights

whatsnew
Added new topics for the SaaS Product Alerts GraphQL API:

• isSubscribedProductAlertPrice — Checks whether the logged-in customer is subscribed to a price drop alert for a specific product.
• isSubscribedProductAlertStock — Checks whether the logged-in customer is subscribed to a stock availability alert for a specific product.
• subscribeProductAlertPrice — Subscribes the logged-in customer to a price drop alert for a specific product.
• subscribeProductAlertStock — Subscribes the logged-in customer to a stock availability alert for a specific product.
• unsubscribeProductAlertPrice — Unsubscribes the logged-in customer from a price drop alert for a specific product.
• unsubscribeProductAlertPriceAll — Removes all price drop alert subscriptions for the logged-in customer.
• unsubscribeProductAlertStock — Unsubscribes the logged-in customer from a stock availability alert for a specific product.
• unsubscribeProductAlertStockAll — Removes all stock availability alert subscriptions for the logged-in customer.