Add module documentation to json_utils

[Lang-Qiu]

Problem Background

The json_utils module in the VRL standard library lacked documentation comments, which reduced code readability and made maintenance more difficult. Good documentation is essential for open-source projects to help contributors and users understand the codebase.

Changes Made

Added a module-level documentation comment to src/stdlib/json_utils/mod.rs to describe the module's purpose: it provides utility modules for BOM (Byte Order Mark) handling and JSON type definitions. This is a minimal, non-functional change that aligns with Rust documentation best practices.

Verification

This change is purely additive and does not affect functionality or API. It can be verified by:

• Reviewing the updated code for clarity and correctness.
• Running existing tests (e.g., cargo test) to ensure no regressions, as only comments were added.