tiup-playground: add info about TiProxy

[qiancai]

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• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
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What is the related PR or file link(s)?

• This PR is translated from: tiup-playground: add info about TiProxy docs-cn#19357
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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

[lilin90]

/lgtm

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-12-23 06:52:14.5481297 +0000 UTC m=+1458124.636932237: ☑️ agreed by lilin90.

[ti-chi-bot]

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[lilin90]

LGTM

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #19771.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #19772.