docs: add LATERAL derived tables documentation

[qiancai]

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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Auto-sync completed successfully

Source PR: pingcap/docs#23123
Target PR: #21757

Chinese documentation has been updated based on English documentation changes.

[qiancai]

docs-cn/keywords.md

Line 396 in a01a708

- LATERAL (R)

The existing keyword.md doc already contains "LATERAL (R)", so there is no need to add it in this PR.

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