docs(java): expand configuration reference

[zhongkechen]

Summary

• document every current Java DurableConfig builder option and its default
• explain backend polling strategies and checkpoint batching tradeoffs
• document serialization round-trip validation and its opt-out behavior
• describe explicit and dynamic Java plugin registration, provider discovery, and layer packaging

Testing

• /tmp/durable-docs-should-complete-venv/bin/mdformat --check docs/
• /tmp/durable-docs-should-complete-venv/bin/codespell docs/
• python3.14 scripts/check_example_refs.py
• python3.14 scripts/vendor_mermaid.py --check
• python3.14 scripts/vendor_glightbox.py --check
• /tmp/durable-docs-should-complete-py314/bin/zensical build --clean
• hidden Unicode and internal-link checks
• verified all documented DurableConfig and PollingStrategies methods against Java main