tiproxy: add docs for label-based balance

[djshow832]

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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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-11-01 08:33:00.004451934 +0000 UTC m=+597892.843607488: ☑️ agreed by hfxsd.
• 2024-11-01 08:59:42.941356773 +0000 UTC m=+599495.780512320: ☑️ agreed by Oreoxmt.

[Oreoxmt]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.3: #18971.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.4: #18972.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18973.