[doc](opt) Add mv task job mv_info opt docs

[seawinde]

Versions

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

Languages

• Chinese
• English
• Japanese candidate translation needed
  • Japanese docs are report-only in the docs governance check and require human review before merging a candidate translation, so they are not included in this PR.

Docs Checklist

• Checked by AI
• Test Cases Built
  • Docs-only change; no site build or runtime test cases were run.
• Updated required version and language counterparts, or explained why not
• If only one language changed, confirmed whether source/translation counterparts need sync

Summary

• Updated materialized-view related jobs, tasks, and mv_infos table-valued function docs across current/dev, 4.x, 3.x, and 2.1 in English and Chinese.
• Added clearer MV job/task/info field descriptions, enum explanations, and example outputs.
• Documented the Doris 2.1.x tasks("type"="mv") limitation: LastQueryId is not available in 2.1.x.
• Clarified the Doris 2.1.x mv_infos partition info limitation: partitioned async materialized views are supported in 2.1.x, but MvPartitionInfo.partitionType = EXPR is supported since Doris 3.0.0; 2.1.x supports FOLLOW_BASE_TABLE and SELF_MANAGE.
• Kept EnvInfo in versioned mv_infos docs because these versioned pages still expose it.

Validation

• git diff --check: passed
• yarn docs:links:changed: no findings
• yarn docs:sql-functions:changed: no findings
• yarn docs:i18n-sync:changed: non-blocking notices/warnings only; current docs are already updated in this PR and Japanese remains report-only.
• yarn docs:lint:changed: exit 0; reports non-blocking SEO/i18n warnings for existing versioned-title/description patterns and report-only Japanese candidates.
• Checked Apache Doris source: 2.1.0-release has partitioned MTMV syntax/code/tests, while MTMVPartitionInfo.MTMVPartitionType.EXPR appears in branch-3.0 and later.

[dataroaring]

LGTM