docs: add objbench subcommand documentation for datanode

[Copilot]

What's Changed in this PR

Documents the new objbench subcommand for datanode, added in greptimedb#7114. This subcommand benchmarks object storage read/write performance for specific SST files.

Added documentation:

• Subcommand purpose and options (--config, --source, --verbose, --pprof-file)
• Basic benchmark example
• Profiling example with flamegraph generation

Examples:

# Basic benchmark
greptime datanode objbench --config ./datanode.toml \
  --source data/greptime/public/1024/1024_0000000000/metadata/8fb41bc7-a106-4b9e-879b-392da799f958.parquet

# With profiling
greptime datanode objbench --config ./datanode.toml \
  --source data/greptime/public/1024/1024_0000000000/metadata/8fb41bc7-a106-4b9e-879b-392da799f958.parquet \
  --pprof-file=./flamegraph.svg

Updated both English and Chinese docs in current (unreleased) version only, as instructed.

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update docs for feat: objbench sub command for datanode</issue_title>
<issue_description>A document change request is generated from GreptimeTeam/greptimedb#7114</issue_description>

<agent_instructions>不要修改最新稳定版的文档</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Update docs for feat: objbench sub command for datanode #2178

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