tiproxy: add vip configurations

[Oreoxmt]

What is changed, added or deleted? (Required)

1. Add virtual IP-related doc.
2. Remove Prometheus-related limitations in load balance. If Prometheus is unavailable, TiProxy will query TiDB directly.

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).

• master (the latest development version)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v8.0 (TiDB 8.0 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)
• v5.2 (TiDB 5.2 versions)

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

• This PR is translated from: tiproxy: add vip configurations docs-cn#18079
• 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]

/cc @djshow832 @qiancai

[ti-chi-bot]

@djshow832: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-12 10:27:25.877511431 +0000 UTC m=+177330.580981074: ☑️ agreed by qiancai.

[Oreoxmt]

/approve
/unhold

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Oreoxmt

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• OWNERS [Oreoxmt]

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[Oreoxmt]

/unhold