docs: add example documentation for raw JSON scalar (issue #427)

[juanscasado]

This PR adds a new documentation page, json-scalar-example.md, that demonstrates how to implement and use a raw JSON scalar in Lacinia.

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[juanscasado]

Description:

This PR adds a new documentation page, json-scalar-example.md, that demonstrates how to implement and use a raw JSON scalar in Lacinia.

Highlights:

Motivation for supporting a raw JSON scalar in GraphQL APIs.
Schema definition in EDN.
Clojure code examples for parsing and serializing JSON using Cheshire.
Sample GraphQL query and response.
Recommendations and warnings.
Additional SDL schema example.
This contribution addresses issue #427 and aims to help users who frequently ask about supporting arbitrary JSON data in their GraphQL APIs.