Add latency formula

[you06]

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Add a document about query latency breakdown.

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• [ ] v5.1 (TiDB 5.1 versions)
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[ti-chi-bot]

[REVIEW NOTIFICATION]

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• Oreoxmt
• ran-huang

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[you06]

@cfzjywxk @sticnarf can you give some advice on this document?

[you06]

I added the railroad diagrams and here is a screenshot of doc page.

@cfzjywxk @sticnarf PTAL again.

[you06]

@shichun-0415 PTAL

[Oreoxmt]

@you06 @ran-huang Preview link is here

[Oreoxmt]

/merge

[ti-chi-bot]

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

In response to this:

/merge

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[Oreoxmt]

/merge

[ti-chi-bot]

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

Commit hash: 321d6f6fb3ead578bca76005ecad35fa035ccfc7