docs: add pnpm command to webpack 5 migration guide #7023

Job	Run time
E2E test and link check (ubuntu-latest, lts/*)	8m 2s
Unit Test (ubuntu-latest, lts/*)	45s
Lint (ubuntu-latest, lts/*)	1m 12s
Proselint (ubuntu-latest, lts/*)	41s
	10m 40s

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