revert(docs): partially revert 326a1d1862872955cec062030df2bd103799a1eb (#5095)

We tried to switch from `yarn` to `npm` because `yarn` ignores lockfiles
but learned that we missed a few key things.

For now, we are reverting docs and a few other changes that suggested
using `npm` instead of `yarn` until we fully remove `yarn` from the
codebase.
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