Update manage-cluster-faq.md for 预估单库大小

[yi888long]

添加了预估单库大小的估算方法，麻烦帮忙查看是否合理，可以添加，非常感谢。
顺便添加了估算后的大小是 mb，在别名里修改了。

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[ti-chi-bot]

Hi @yi888long. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ti-chi-bot]

Welcome @yi888long!

It looks like this is your first PR to pingcap/docs-cn 🎉.

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Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ hfxsd
❌ yi888long
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[hfxsd]

/ok-to-test

[hfxsd]

/bot-review

[github-actions]

✅ AI review completed, 7 comments generated.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot]

@yi888long: 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	ea99831	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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