perf-tuning: Add document for topn-limit-push-down (#3307)

[ti-srebot]

cherry-pick #3307 to release-3.0

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What is changed, added or deleted? (Required)

Add Chinese document for topn-limit-push-down.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

[ti-srebot]

@LittleFall please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/docs-cn/invitations

[yikeke]

@LittleFall seems on leave recently. Could you help fix the lint and add this file to TOC.md? @zz-jason

[LittleFall]

@yikeke sorry this lint makes me confused.
when I add the slash like [优化规则及表达式下推的黑名单](/blacklist-control-plan.md), it finds a dead link: /home/circleci/pingcap/docs-cn//blacklist-control-plan.md as lint(19480).
when I remove the slash like [优化规则及表达式下推的黑名单](blacklist-control-plan.md), it says link-must-start-with-slash:blacklist-control-plan.md as lint(19479).

[yikeke]

@yikeke sorry this lint makes me confused.
when I add the slash like [优化规则及表达式下推的黑名单](/blacklist-control-plan.md), it finds a dead link: /home/circleci/pingcap/docs-cn//blacklist-control-plan.md as lint(19480).
when I remove the slash like [优化规则及表达式下推的黑名单](blacklist-control-plan.md), it says link-must-start-with-slash:blacklist-control-plan.md as lint(19479).

/blacklist-control-plan.md is the correct format. But the blacklist-control-plan.md doc has not been merged to release-3.0 branch, so there's this dead link and this PR should be merged after #3760 is merged.

[yikeke]

@yikeke sorry this lint makes me confused.
when I add the slash like [优化规则及表达式下推的黑名单](/blacklist-control-plan.md), it finds a dead link: /home/circleci/pingcap/docs-cn//blacklist-control-plan.md as lint(19480).
when I remove the slash like [优化规则及表达式下推的黑名单](blacklist-control-plan.md), it says link-must-start-with-slash:blacklist-control-plan.md as lint(19479).

/blacklist-control-plan.md is the correct format. But the blacklist-control-plan.md doc has not been merged to release-3.0 branch, so there's this dead link and this PR should be merged after #3760 is merged.

#3760 is merged and the lint should pass now. Please add this file to TOC.md. @LittleFall

[ti-srebot]

@ti-srebot, please update your pull request.