*: add document for column-level masking policy feature

[tiancaiamao]

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• This PR is translated from:
• Other reference link(s): *: add column-level masking policy feature document docs-cn#21454

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces comprehensive documentation for the new column-level masking policy feature in TiDB. This feature is designed to enhance data security and compliance by allowing sensitive data to be masked at the column level, with masking rules configurable based on user roles and privileges. The documentation covers the feature's functionality, management, and various masking options, ensuring users can effectively implement and manage data protection.

Highlights

• New Documentation for Column-Level Masking Policy: A comprehensive document has been added to explain the new column-level masking policy feature in TiDB, enhancing data security and compliance.
• Detailed Feature Explanation: The new documentation provides an in-depth overview of the masking policy, including its purpose, key characteristics, and benefits for protecting sensitive data.
• Policy Management and Syntax: Instructions are provided for creating, modifying, enabling, disabling, and dropping masking policies, along with the necessary SQL syntax and required privileges.
• Built-in Masking Functions: Four dedicated built-in functions (MASK_PARTIAL, MASK_FULL, MASK_NULL, MASK_DATE) are detailed, offering various methods for data redaction with examples for different data types.
• Conditional Masking and Restrictions: The document explains how to implement conditional masking based on user identity (current_user()) and roles (current_role()), and how to use the RESTRICT ON clause to prevent data exfiltration during specific SQL operations.
• Behavioral Considerations and Limitations: Key aspects such as at-result masking, supported column types, and current limitations (e.g., no masking on views or temporary tables) are clearly outlined.
• Table of Contents Update: The main Table of Contents (TOC.md) has been updated to include a direct link to the new column-level masking policy documentation.

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[gemini-code-assist]

Code Review

This pull request introduces comprehensive documentation for TiDB's new Column-Level Masking Policy feature, including its overview, required privileges, syntax for creation and management, details on built-in masking functions (MASK_PARTIAL, MASK_FULL, MASK_NULL, MASK_DATE), conditional masking based on user/role, RESTRICT ON semantics, behavior considerations, and MySQL compatibility notes. The review comments suggest improvements to enhance the documentation's clarity, maintain an active voice, and ensure consistent markdown formatting.

[tiancaiamao]

Ref pingcap/tidb#65744

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[ti-chi-bot]

@tiancaiamao: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	601198b	link	true	/test pull-verify

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