add notice about scaling in pd node (#6099)

[ti-chi-bot]

This is an automated cherry-pick of #6099

What is changed, added or deleted? (Required)

Add a notice about scaling PD nodes, this is the same as in "scale tidb cluster with Ansible" document.

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: add notice about scaling in pd node docs-cn#6514
• 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]

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

Commit hash: 804c3e2

[Liuxiaozhen12]

/merge