docs: document two-phase series scan mode

[evenyag]

What changed

• Document the region_engine.mito.experimental_series_scan_v2 option in the English and Chinese Nightly configuration guides.
• Document the SeriesScan.mode field and its two_phase and legacy values in the English and Chinese EXPLAIN references.

Closes #2714.

Scope

• Documentation versions: Nightly
• Languages: English, Chinese

The feature merged after the v1.2 documentation cut, so released documentation is unchanged.

Verification

• git diff --check
• Compiled all four changed Markdown files with @mdx-js/mdx
• pnpm build and DOC_LANG=zh pnpm build were attempted, but the host exhausted its process table and Docusaurus failed while spawning Git history lookups for unrelated pages with EAGAIN.

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; this change adds none.
• I updated navigation when the document structure changed; navigation is unchanged.

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[fengjiachun]

LGTM