new nav: move ddl-introduction.md to /best-practices and java-app-best-practices.md to /develop

[qiancai]

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• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

• Relocated 'ddl-introduction.md' to 'best-practices/ddl-introduction.md' and 'java-app-best-practices.md' to 'develop/java-app-best-practices.md'.
• Updated all relevant documentation links and TOC entries to reflect the new locations.
• Cleaned up TOC files by removing or adjusting references to these documents where appropriate.

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-22 09:54:53.404714463 +0000 UTC m=+660521.018671309: ☑️ agreed by lilin90.

[qiancai]

/approve

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[lilin90]

Note

The corresponding docs-cn updates are covered in the following PRs:

• new nav: add TOC and doc files for AI and Developers docs-cn#21294
• *: create best practices TOC and index docs-cn#21296