docs: update Flow user guide

[discord9]

What changed

• update the Nightly Flow user guide to match current batching and scheduled full-query behavior
• document EVAL INTERVAL, planner-valid joins, subqueries, SQL CTEs, missing-source deferral, incremental reads, and Flow runtime inspection
• clarify sink schema and update_at behavior for SQL and TQL flows
• correct EXPIRE AFTER scope, stale SQL examples, cleanup commands, and English/Chinese wording

Verified against GreptimeDB upstream/main at 144f83528db8648bc5607fdc65c249bbfa3c7ffc.

Scope

• Documentation versions: Nightly
• Languages: English, Chinese

Verification

• git diff --check
• pnpm build
• DOC_LANG=zh pnpm build
• DOC_LANG=en pnpm check:links
• DOC_LANG=zh pnpm check:links
• independent source-based review of the eight-file diff

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.
• I updated navigation when the document structure changed. No navigation change was required.