[docs](variant) make reference page the navigation root

[eldenmoon]

Summary

• make the VARIANT reference page (semi-structured/VARIANT) the VARIANT navigation root
• move the workload/tuning guide (variant-workload-guide) under it as VARIANT Internal
• apply the same hierarchy to current, 4.x, and 3.x without changing either document route

Validation

• git diff --check origin/master...HEAD
• parsed the 4.x and 3.x versioned sidebar JSON files
• asserted that each VARIANT root links to the reference page and its only child links to the workload guide

Versions

• dev
• 4.x
• 3.x
• 2.1 or older (not covered by version/language sync gate)

Languages

• Chinese
• English
• Japanese candidate translation needed

Docs Checklist

• Checked by AI
• Test Cases Built
• Updated required version and language counterparts, or explained why not
• If only one language changed, confirmed whether source/translation counterparts need sync

The hierarchy is defined in the shared sidebar configuration; no localized document content or routes changed.

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