docs: document parquetbench usage

[evenyag]

What changed

Add a parquetbench section to the Datanode CLI reference, covering local SST benchmarking with --file-path, object-store mode, reader and argument restrictions, column/row-group selection, and profiling after warmup. Explain that the SST must contain GreptimeDB region metadata.

This PR documents only parquetbench from GreptimeTeam/greptimedb#8939, as requested for this issue.

Closes #2780

Scope

• Documentation versions: Nightly
• Languages: English, Chinese

Local-file support is not included in the existing 1.2 or older documentation releases. This extends existing pages and requires no navigation changes.

Verification

• DOC_LANG=en pnpm check:links (includes the English production build)
• DOC_LANG=zh pnpm check:links (includes the Chinese production build)
• git diff --check
• Verified options, defaults, input restrictions, projection/row-group handling, and warmup behavior against upstream source.
• Parsed the JSON example and checked that English and Chinese code examples match.
• No benchmark was run against a deployment.

Checklist

• I verified the content against the applicable GreptimeDB version.
• I updated the relevant documentation versions and languages, or explained why not.
• I checked changed links and anchors.
• Navigation does not require an update because this extends an existing page.

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