Add API NPM endpoints to OpenApiV2 documentation

🔍 What does this MR do and why?

• Add OpenApiV2 documentation to API::NpmProjectPackages and API::NpmProjectPackages.

See https://gitlab.com/gitlab-org/gitlab/-/issues/376949 and https://gitlab.com/gitlab-org/gitlab/-/issues/376948.

📺 Screenshots or screen recordings

⚙ How to set up and validate locally

1. Run rake gitlab:openapi:generate
2. Review changes in doc/api/openapi/openapi_v2.yaml. You can check the content of the file by pasting it into https://editor.swagger.io.

⚠ Be aware of !105498 (comment 1192248378).

🚥 MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.