tiup: add note for performance testing on TiUP Playground

[Oreoxmt]

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What is changed, added or deleted? (Required)

tiup: add note for performance testing on TiUP Playground

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• [x] master (the latest development version)
• [x] v8.4 (TiDB 8.4 versions)
• [x] v8.3 (TiDB 8.3 versions)
• [x] v8.2 (TiDB 8.2 versions)
• [x] v8.1 (TiDB 8.1 versions)
• [x] v7.5 (TiDB 7.5 versions)
• [ ] v7.1 (TiDB 7.1 versions)
• [ ] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)
• [ ] v5.3 (TiDB 5.3 versions))

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• [ ] Delete files
• [ ] Change aliases
• [ ] Need modification after applied to another branch
• [ ] Might cause conflicts after applied to another branch

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[qiancai]

Removed the needs-cherry-pick-release-8.2 label because the v8.2 docs have been archived at https://docs-archive.pingcap.com/zh/tidb/v8.2 and will no longer receive new updates.

[qiancai]

Removed the needs-cherry-pick-release-8.3 label because the v8.3 docs have been archived at https://docs-archive.pingcap.com/zh/tidb/v8.3 and will no longer receive new updates.