tiproxy: add docs for label-based balance (#18914)

[ti-chi-bot]

This is an automated cherry-pick of #18914

What is changed, added or deleted? (Required)

Label-based balance was released in v1.1.0, but it was not described in the docs because we may need to design it carefully but there were no users at that time, so we couldn't make sure if it was well-designed.

Recently, 2 community users mentioned it and want this feature (pingcap/tiproxy#609 and https://asktug.com/t/topic/1033941). The current design can satisfy their needs, so it's time to expose it to users.

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• master (the latest development version)
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• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
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• 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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Do your changes match any of the following descriptions?

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

[Oreoxmt]

/approve

[ti-chi-bot]

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