docs/windows: Dedupe and align with main docs in using `yarn`.
docs/windows: Dedupe and align with main docs in using `yarn`.

We don't recommend running `npm` anywhere else in our developer
documentation; no need to do so here.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile