docs: update Flow user guide

[discord9]

What changed

• Refresh the Flow user guide, clarify batching versus scheduled full-query execution, and correct SQL examples and cleanup commands.
• Document missing-source deferral and the manual drop/recreate activation workflow, append-only experimental incremental reads, sink schema, and SQL/TQL update_at differences.
• Present Flow inspection commands and their purposes as tables in English and Chinese.
• Add the required evaluation interval to the non-windowed quick-start aggregate in all three versions.
• Backport release-valid guidance to v1.1 and v1.2. Keep their older mode-selection behavior: EVAL INTERVAL does not force batching there, while current Nightly does after feat(flow): support eval schedule offsets greptimedb#8878.
• Omit SHOW FLOW STATUS and information_schema.flow_statistics from v1.1 because those interfaces are unavailable in v1.1.4.

Source verification: GreptimeDB 27b0ca667634e132a13f29ff268f8b2472295bee for current Nightly behavior, v1.1.4, and v1.2.0-beta.2.

Scope

• Documentation versions: Nightly, v1.1, v1.2
• Languages: English, Chinese
• 24 Flow user-guide pages; no configuration, navigation, lockfile, or generated-file changes relative to the base branch.

Verification

• git diff --check origin/main...HEAD
• DOC_LANG=en pnpm check:links (includes production build)
• DOC_LANG=zh pnpm check:links (includes production build)
• Source-based independent reviews of Nightly and release-specific behavior
• EN/ZH inspection-command parity and generated HTML table checks

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.

[killme2008]

LGTM

Don't forget to backport them into v1.2 and v1.1 if possible.

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