Update docs for feat: add a subcommand to bench scan

[sunng87]

A document change request is generated from GreptimeTeam/greptimedb#7722

[evenyag]

@codex update docs

[chatgpt-codex-connector]

Summary

• Added a new scanbench section to the Datanode CLI reference, including purpose, full option table, scan-config JSON shape, and usage notes. docs/reference/command-lines/datanode.mdL88-L127
• Added practical scanbench command examples for sequential, unordered (parallel), and series scan modes. docs/reference/command-lines/datanode.mdL128-L146
• Kept the existing objbench section intact while extending the same page with the new subcommand documentation so users can find benchmarking-related commands in one place. docs/reference/command-lines/datanode.mdL55-L90

Testing

• ✅ git diff --check
• ❌ npx --yes prettier --check docs/reference/command-lines/datanode.md (initially failed due formatting issues detected in the edited file)
• ✅ npx --yes prettier --write docs/reference/command-lines/datanode.md
• ✅ npx --yes prettier --check docs/reference/command-lines/datanode.md

Committed on branch work with message:
docs: add datanode scanbench command reference

Also created a follow-up PR record via make_pr with title:
docs: document datanode scanbench subcommand usage.

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