tiup: add notice about importing cluster (#6108)

[ti-chi-bot]

This is an automated cherry-pick of #6108

What is changed, added or deleted? (Required)

Some clusters deployed by TiDB-Ansible may not having monitoring agents deployed on all nodes, and if the user imported the cluster with TiUP Cluster, future operations may fail.

This PR adds a notice to the import doc to guide the user to deploy monitoring agents to all nodes before importing to TiUP Cluster.

This PR changes the manual page of tiup cluster import subcommand, I'll file another PR to change the import guidance doc in the release-4.0 branch.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

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

• This PR is translated from: tiup: add notice about importing cluster that don't have all nodes mo… docs-cn#6659
• 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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Liuxiaozhen12

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[Liuxiaozhen12]

/merge

[ti-chi-bot]

@Liuxiaozhen12: /merge in this pull request requires 1 approval(s).

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In response to this:

/merge

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[Liuxiaozhen12]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

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Commit hash: cb5966f