fix(docs): use `pnpm add <pkg>` instead of `pnpm install <pkg>` (#10529)
It should be `pnpm add <pkg>` instead of `pnpm install <pkg>`.
`pnpm install` is used to install all dependencies for a project while
`pnpm add <pkg>` is used to install a package and any packages that it
depends on.
More info (official source):
- https://pnpm.io/cli/add
- https://pnpm.io/cli/install
---
`pnpm install <pkg>` does the same as `pnpm add <pkg>` today, but it
might stop functioning that way in future versions since the official
docs clarify the difference between the two. It might be that `pnpm
install <pkg>` was used in past versions, and the PNPM team didn't want
to remove support for it right away after introducing `pnpm add <pkg>`.