docs: add manual package-manager install path for vp #6537

Job	Run time
optimize-ci	4s
detect-changes	5s
download-previous-rolldown-binaries	0s
Test	0s
Lint	0s
Run task	0s
done	2s
CLI E2E test	0s
Local CLI `vp install` E2E test	0s
	11s

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