Docs: add Yarn

[jessicalins]

When trying to build locally I got:

jessica@cupsofwonder:~/workspace/parca|main ⇒  make build
make: Circular ui/packages/app/web/dist <- ui dependency dropped.
cd ui && yarn install && yarn workspace @parca/web build
/bin/sh: 1: yarn: not found
make: *** [Makefile:51: ui/packages/app/web/dist] Error 127

This just adds a link to the docs referring to Yarn too :)

Signed-off-by: Jéssica Lins jlins@redhat.com

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[brancz]

Thank you!