add tiflash execution tuning materials

[shichun-0415]

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

• This PR is translated from: Add tiflash execution tuning materials docs-cn#12637, Correct the unit of tidb_max_tiflash_threads docs-cn#12664
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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[yibin87]

LGTM

[TomShawn]

mostly minor comments

[TomShawn]

LGTM

[shichun-0415]

/remove status LGT1
/status LGT2

[shichun-0415]

/merge

[ti-chi-bot]

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

Commit hash: 835e5b9

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #12048.