docs(java): replace the Java API example that no longer compiles

[jackylee-ch]

The only Java example on the docs site cannot compile: dev.vortex.api.File and
dev.vortex.api.Array were removed in #7527, and the page has not been touched since. Nothing in
it points at the API that replaced them.

Replaced with the read path that exists today — Session → DataSource.open → scan →
Partition.scanArrow — following the same loop the JNI tests use.

Tests

Compiled both snippets as a scratch class under vortex-jni/src/test: the published one fails with
five cannot find symbol errors for File and Array, the new one compiles. make -C docs html
succeeds under the repo's --fail-on-warning.

AI assistance

Written with agentic AI assistance; I compiled the replacement snippet before publishing it.

[codspeed-hq]

Merging this PR will improve performance by 33.22%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚡ 3 improved benchmarks
✅ 2109 untouched benchmarks
⏩ 106 skipped benchmarks¹
🗄️ 4 archived benchmarks run²

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	arrow_checked_add_u32_neon[16384]	20.4 µs	12.3 µs	+65.35%
⚡	WallTime	arrow_checked_add_u32_avx512[16384]	21.3 µs	17.6 µs	+20.78%
⚡	Simulation	take[duplicates/repeated/primitive/nonnull/chunks=16/indices=1000]	245 µs	207 µs	+18.39%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing jackylee-ch:java-api-docs-example (961d17f) with develop (ee1ac25)}

Footnotes

1. 106 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. 4 benchmarks were run, but are now archived. If they were deleted in another branch, consider rebasing to remove them from the report. Instead if they were added back, click here to restore them. ↩