Document usage of JSONEachrow reader

## Summary

Document briefly what is the idea and where to find examples
Define responsibilities
Note that feature is in beta

Also look into this comment:

JacksonJsonParserFactory builds a default ObjectMapper with no special number policy, while GsonJsonParserFactory uses ToNumberPolicy.BIG_DECIMAL so UInt64 and decimal JSON values are not materialized as Double. With JSON_DISABLE_NUMBER_QUOTING, Jackson-backed readers can lose precision or infer the wrong types for large integers and decimals.

首先定位 issue 中提到的 JSONEachrow reader、JacksonJsonParserFactory 和 GsonJsonParserFactory，然后阅读它们现有的使用示例和职责。完成的标准是编写简要文档，涵盖 reader 的用途、示例、职责、beta 状态，以及评论中提出的数字处理问题。

由索引模型根据 Issue 内容生成。