add doc for tiflash resource control

[guo-shaoge]

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

• This PR is translated from: add doc for tiflash resource control docs-cn#15043
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
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• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-09-26 07:39:57.189776215 +0000 UTC m=+346986.908118435: ☑️ agreed by Oreoxmt.
• 2023-09-28 07:44:48.07313971 +0000 UTC m=+87885.660249855: ☑️ agreed by ran-huang.

[Oreoxmt]

/approve

[ti-chi-bot]

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