Update the endpoint for getMenu

[bojanbogdanovic]

This pull request is for: (mark with an "x")

• [ ] examples/*
• [ ] modules/next
• [x] packages/next-drupal
• [ ] starters/basic-starter
• [ ] starters/graphql-starter
• [ ] starters/pages-starter
• [ ] Other

GitHub Issue: #752

Describe your changes

As described in the related issue, menu_items is not an entity and therefore is not exposed in the /jsonapi index. Instead of creating a menu_items endpoint as a resource type, build the endpoint using its path.

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