docs: update PackageManagerTabs tips for pnpm

[Marukome0743]

Summary

Documentation:

• Mention pnpm alongside yarn and bun for special install→add processing in PackageManagerTabs.

Related Issue

Follow up #2364.
I forgot to update document.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Marukome0743]

@JounQin
Could you re-run the failed test?
it seems to be flaky.