docs: update the default auto flush interval

[evenyag]

What changed

Update the Mito configuration example and default-value reference to use a 10-minute auto-flush interval. Clarify that explicit engine configuration and per-table settings override the default.

Verified against GreptimeTeam/greptimedb#8971, which changes the runtime default from 30 minutes to 10 minutes and replaces the old 1-hour example configuration.

Closes #2798

Scope

• Documentation versions: Nightly
• Languages: English, Chinese

The upstream change is not included in the existing 1.2 or older documentation releases.

Verification

• pnpm build
• DOC_LANG=zh pnpm build
• DOC_LANG=en pnpm check:links
• DOC_LANG=zh pnpm check:links
• git diff --check
• Compared the runtime default, example configuration, and override semantics with upstream source.

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.
• Navigation does not require an update because this changes an existing page.

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[killme2008]

@evenyag Merged. If a backport is needed, please open a separate PR.