Add information about expected GraphQL format to API Security docs

What does this MR do and why?

Provides information about the GraphQL schema input file format and link to a conversion tool.

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.

Related to #373763 (closed)