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.

Start by locating the JSONEachrow reader and the JacksonJsonParserFactory and GsonJsonParserFactory mentioned in the issue, then read their existing usage examples and responsibilities. Done means brief documentation covering the reader's purpose, examples, responsibilities, beta status, and the number-handling concern raised in the comment.

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