br: add compact log backup

[YuJuncen]

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What is changed, added or deleted? (Required)

This added docs to the new feature compact log backup. (experimental)

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For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
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• 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)

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Do your changes match any of the following descriptions?

• Delete files
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• Need modification after applied to another branch
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[ti-chi-bot]

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[ti-chi-bot]

@YuJuncen: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	960141a	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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