Updates for Customer Group required header description

[meker12]

Purpose of this pull request

Updates description for Magento-Customer-Group header required for Catalog Service and Product Recommendations API requests.

DATA-6141

Preview of updated information rendered in the Required Header table and explanation of finding customerGroupCode.

Affected pages

• https://developer.adobe.com/commerce/services/graphql/catalog-service/products/#required-headers
• https://developer.adobe.com/commerce/services/graphql/recommendations/recommendations/#required-headers
• https://developer.adobe.com/commerce/services/graphql/live-search/product-search/#required-headers