tiproxy: add a guide to enable tiproxy using tiup

[djshow832]

What is changed, added or deleted? (Required)

1. More and more users encountered a problem that after scaling out a tiproxy, tiproxy can't migrate sessions but reports warnings. This doc adds a guide to scale out tiproxy on an existing cluster.
2. Before TiUP v1.16.1, there were some bugfixes related to tiproxy, refer to https://github.com/pingcap/tiup/releases. Besides, adding deployment with TiUP v1.15.0 makes the steps longer. This doc removes the TiUP v1.15.0 steps and recommends v1.16.1.

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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).

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
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• v8.4 (TiDB 8.4 versions)
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• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
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• Delete files
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• Need modification after applied to another branch
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Timeline:

• 2025-04-17 10:07:06.896288374 +0000 UTC m=+2942120.580524470: ☑️ agreed by Oreoxmt.
• 2025-04-18 02:29:16.640136935 +0000 UTC m=+3001050.324373030: ☑️ agreed by hfxsd.

[hfxsd]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #20237.