sql-plan: add description about plan baseline

[hfxsd]

What is changed, added or deleted? (Required)

as tittle

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What is the related PR or file link(s)?

• This PR is translated from:sql-plan: add description about plan baseline docs-cn#13769
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• Delete files
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[ti-chi-bot]

[REVIEW NOTIFICATION]

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[hfxsd]

/merge

[ti-chi-bot]

@hfxsd: /merge in this pull request requires 2 approval(s).

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/merge

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[hfxsd]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 7f34efb