Document Java External Storage

[lennessyy]

Summary

• add the Java SDK External Storage guide, including Amazon S3 setup and a runnable custom-driver example
• document payload size thresholds, multiple-driver migration, and S3 multi-region durability with CRR and MRAP
• add the guide to Java data handling navigation and the External Storage encyclopedia

Dependency

This draft uses snippets from temporalio/features#904. Merge that PR before marking this one ready for review.

Validation

• compiled all source snippets against released Java SDK v1.39.0 dependencies
• exercised the custom driver store/retrieve contract
• ran a Java Client → Workflow → Activity → Client S3 round trip against a temporary private AWS bucket; verified two stored objects and removed the bucket afterward
• vale --config .vale-ci.ini (0 errors, 0 warnings)
• yarn build passed
• yarn check:orphans still reports two unrelated pre-existing Cloud comparison baseline issues

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Project	Deployment	Actions	Updated
temporal-documentation	Ready	Preview	Sep 15, 2026 8:26pm UTC

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📖 Docs PR preview links

• Develop
  • Java
    • Best Practices
      • Data handling
    • Data Handling
      • External Storage
• External Storage