tiproxy: add docs for label-based balance (#19288)

[ti-chi-bot]

This is an automated cherry-pick of #19288

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 (https://github.com/pingcap/tiproxy/issues/609 and https://asktug.com/t/topic/1033941). The current design can satisfy their needs, so it's time to expose it to users.

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)
• [ ] v8.1 (TiDB 8.1 versions)
• [ ] 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)

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

• This PR is translated from: https://github.com/pingcap/docs-cn/pull/18914
• Other reference link(s):

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[bot]]

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