fix(docs): use `pnpm add <pkg>` instead of `pnpm install <pkg>` #5833

Job	Run time
Find path changes	18s
`kitchen-sink` example	4m 34s
`with-tailwind` example	4m 2s
`basic` example	3m 54s
Turborepo Rust testing on ${{ matrix.os.name }}	0s
`with-svelte` example	3m 0s
`non-monorepo` example	4m 30s
Turborepo Integration	0s
Rust lints	0s
Turborepo rust check	0s
Turborepo Test Summary	4s
Cleanup / cleanup	4s
	20m 26s

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