[DNM]: Support database read-only

[fzzf678]

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[gemini-code-assist]

Summary of Changes

Hello @fzzf678, 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 new documentation for the database-level read-only feature in TiDB. The document comprehensively covers the functionality, syntax, and practical applications of setting a database to read-only, including its impact on DDL and DML operations, interactions with various tools, and observability. It also details important compatibility considerations with MySQL 8.0, providing users with a complete guide to leveraging this new capability for scenarios like data migration and failback.

Highlights

• Introduction of Database-Level Read-Only: This pull request introduces comprehensive documentation for a new feature that allows individual databases to be set to read-only mode, effectively preventing write operations (DDL, DML, and some locking reads) against them.
• Comprehensive Usage Guide: The documentation provides detailed syntax for the ALTER DATABASE ... READ ONLY statement, outlines practical usage scenarios such as sharded migration and data export, and specifies required privileges and restrictions on system databases.
• Operational Behavior and Observability: It explains the behavior of DDL transactions when switching read-only states, lists all rejected and allowed operations within a read-only database, and describes how to observe the read-only status using INFORMATION_SCHEMA.SCHEMATA_EXTENSIONS and SHOW CREATE DATABASE.
• MySQL Compatibility Notes: Key differences between TiDB's implementation of database read-only and MySQL 8.0 are highlighted, covering aspects like combining options in ALTER DATABASE, handling of temporary tables, and the timing of read-only checks for PREPARE and EXPLAIN statements.

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[gemini-code-assist]

Code Review

This pull request introduces documentation for the new database-level read-only feature. The document is well-written and covers the syntax, behavior, observability, and MySQL compatibility aspects of the feature. My review provides several suggestions to improve clarity, fix minor grammatical issues, and align the text more closely with the repository's style guide, especially regarding the preference for active voice. These changes aim to make the documentation more direct and easier for users to understand.

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