GraphQL docs: Handling data stored as JSON

What does this MR do?

This MR adds some pointers for how to expose data stored as JSON in our GraphQL API.

The motivation to add this section came from a question posed to #f_graphql and also asked in #231386 (closed) around how to do this.

Author's checklist (required)

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• If you have developer access or higher (for example, GitLab team members or Core Team members)
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Review checklist

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1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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