config,variable: add new SEM configuration document

[YangKeao]

First-time contributors' checklist

• [ ] I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

1. Add a more detailed configuration guide for SEM.

Ref https://github.com/pingcap/tidb/pull/61634

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• [x] master (the latest development version)
• [x] v9.0 (TiDB 9.0 versions)
• [ ] v8.5 (TiDB 8.5 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)

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• [ ] Delete files
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• [ ] Might cause conflicts after applied to another branch

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@YangKeao: 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	d54f4f4f943f50e9a411d5db0a5d9c8e19bd8022	link	true	/test pull-verify

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