docs: add LATERAL derived tables documentation

[terry1purcell]

Document the LATERAL derived table syntax support added in pingcap/tidb#67076. Includes a new page with syntax, examples, and current limitations, plus updates to mysql-compatibility.md and TOC.md.

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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 LATERAL derived tables in TiDB. It provides users with detailed information on this SQL feature, including its syntax, practical examples, and current implementation status, ensuring clarity on its compatibility with MySQL and TiDB's roadmap for full support.

Highlights

• New Documentation Page: A dedicated documentation page for LATERAL derived tables has been added, explaining their syntax, examples, and current limitations in TiDB.
• Table of Contents Update: The main Table of Contents (TOC.md) has been updated to include a link to the new LATERAL derived tables documentation.
• MySQL Compatibility Clarification: The mysql-compatibility.md file has been updated to provide more detailed information on TiDB's support for LATERAL derived tables, including a note on parsing vs. execution support and a link to the new documentation.

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[gemini-code-assist]

Code Review

This pull request introduces new documentation for LATERAL Derived Tables in TiDB, including a dedicated markdown file explaining its concept, syntax, examples, and current support status (parsing only, execution pending). The changes also update the table of contents and the MySQL compatibility document to reference this new content. Review feedback suggests minor stylistic and clarity improvements for the new documentation, such as rephrasing for active voice and better wording for column descriptions.

[qiancai]

A few line-level doc fixes are still needed before merge.

[qiancai]

@terry1purcell Please involve a tech reviewer for this doc PRs. Thanks.