docs: add json_get_object UDF function

[Copilot]

What's Changed in this PR

Documents the new json_get_object(json, path) UDF added in GreptimeTeam/greptimedb#7241.

Function behavior:

• Extracts object values from JSON by path
• Returns NULL if path doesn't point to an object
• Can be chained with other JSON functions for nested access

Example:

SELECT json_to_string(json_get_object(parse_json('{"a": {"b": {"c": {"d": 42}}}}'), 'a.b.c'));
-- Returns: {"d":42}

Updated files:

• docs/reference/sql/functions/json.md - Added function to Extraction section with example
• i18n/zh/docusaurus-plugin-content-docs/current/reference/sql/functions/json.md - Chinese translation

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update docs for feat: udf json_get_object</issue_title>
<issue_description>A document change request is generated from GreptimeTeam/greptimedb#7241</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Update docs for feat: udf json_get_object #2223

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