GH-51395: [C++][Docs] Replace RapidJSON with simdjson in row converter example

[Reranko05]

Rationale for this change

This PR continues the simdjson migration by replacing RapidJSON usage in the C++ row converter example with simdjson and Arrow's existing JsonWriter.

Changes

• Replace RapidJSON DOM parsing with simdjson's DOM API.
• Replace RapidJSON JSON generation with Arrow's JsonWriter.
• Preserve the existing row-to-Arrow and Arrow-to-row conversion flow.
• Update the example to link against arrow::simdjson.

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• GitHub Issue: [C++][Docs] Replace RapidJSON with simdjson in row converter example #51395

[Reranko05]

Will mark this PR Ready to Review after #51038 is merged.